Why is my baguette type bread coming out almost flat?

If you haven't changed your recipe then the problem is in your method, and it sounds like you are over proofing. If your bread expands too much in the final rise it gets over-extended and tends to collapse when scored, exactly as you describe. It's easy to 'get greedy' and try to eke out that much more expansion, but it's actually a very common mistake.

Reduce your final rise after shaping and you'll get a better result. Go for a doubling in size typically, maybe a bit more. It's hard to give a time on that, but typically 1.5 hours is the most I'd give it, I like to still have some visible rising going on when I put it in the oven because part of oven spring comes from the yeast.

If you do over-prove it's not a disaster, just don't score it, then put it into the oven gently.

As for how to proof the dough, you don't need to use the oven, in fact I'd recommend against that unless your house is really cold. 70°f is fine for proofing, it just takes a bit longer than a warm oven, and you often get more flavor and better structure if you don't rush it. One thing I would suggest is not to use a damp towel as the weight of it on a small baguette can hold it down a bit, and the evaporation of water from the towel cools the bread and slows down proofing. Instead proof your bread in a clear plastic bag (clear so you can see through it, no other reason). You blow air into the bag and then clip it, the air will keep the bag from contacting the dough and sticking. The bag will seal in the heat generated from the yeast activity as well as keeping in moisture, preventing it from forming a skin.

It's impossible to know the cause without quizzing you further about various details, but there are a couple of really important things that stop loaves from spreading that you don't mention;

Tension

Firstly, it's really important you shape your bread to create tension across the top of the loaf. This is one of the most easily overlooked factors that stops loaves from spreading. With the loaf upside-down, keep stretching from the sides and folding to the middle until the underside (which will be the top when you bake it) is under a lot of tension. Then turn it over to bake. This makes sure the strands of gluten are stretched out laterally and pulling across the top of the loaf. If your bread is full of gas, but has just spread too much then failure to do this is more likely your problem.

Overproofing

People often talk about over- or under-proofing but rarely give good instructions when is the right moment to catch it. The clearest advice I was ever given is that you need to catch it on the rise. i.e. as the bread is rising, not when it has topped-out. Of course, it needs handling gently once it's risen, but if it feels like it's about to deflate and collapse in response to the slightest disturbance then it's gone too far. If your bread lacks structure especially at the bottom, with all the larger gas bubbles at the top, or there is evidence of bubbles breaking out of the top of your crust, you more likely overproofed it

I got in to bread making years ago, but with the pandemic causing me to spend more time indoors, I was able to spend more time in the kitchen to improve my skills.

Here are the things (from what you've described) that might be affecting your bread's rising process.

1. YEAST: The type and amount of yeast being used. You mention "proofing" so I believe you are probably using active-dry yeast, and because you mention "poolish" I assume that the total amount of yeast being used is much less than a standard loaf of bread recipe.(less than 1 tsp yeast total?) additionally,is your yeast is still active? when you proof it - does it turn in to a lovely bubbly foam? if not - you might want to try a new package of yeast. Active dry yeast must be proofed in water that is warm enough, ideal temp is between 105-115F

2. FLOUR: You mentioned that you mix AP & bread flour together. The reason bread flour is perfect for making bread is due to its high protein content. AP flour has between 8-11% percent protein, while bread flour contains between 12-14% percent. Using AP flour will give you rise, but the extra protein in bread flour will give you a slightly higher rise. Bread flour also produces more gluten, which makes bread just a bit denser and chewier. So I might experiment will using a higher ratio of bread to AP flour - if you can, try 90% bread flour to 10% AP flour. AP flour (ive noticed) is great for fluffier breads, breakfast breads, dinner rolls.

3. USING A PRE-FERMENT (POOLISH) you said that you let the poolish rise for 24 hours - I would cut that down to a max of 15 hours - but checking on it to see what it looks like is your best bet. Poolish is the most hydrated preferment (approx 1 part flour to 1 part water) and while it can start out like a shaggy dough ball, when its ready it looks almost soupy. Ripeness is indicated when the surface is covered with small bubbles and the mass has expanded. Use it once you've notice it's doubled in size and has a bunch of small bubbles - if you wait too long, the size will start to decrease .

4. MAN VS MACHINE: Now, I know that many people love electric mixers, obviously they take a lot of the hard work out of the process, but I honestly feel kneading dough by hand is the way to go, you can literally feel the integrity of the dough build with each pass. Kneading is important because its how you make the dough form gluten which makes the bubbles that develop able to withstand more pressure as the mass grows taller. Are you doing the "windows" test before letting it set for the first rise?

5. TEMPERATURE: letting the dough rise in an environment thats at least 80F will be most ideal to support rise. try using a slightly damp tea towel over it too.

6. RISING/ CUTTING/ SHAPING If you intend on cutting the risen dough to make multiple loafs, after the 1st rise, carefully dump the dough on to a floured surface and use a dough scrapper to divide and move each piece to be shaped. Let the shaped loaves rise a 2nd time until doubled in size, then bake :)

Try THIS recipe, it has worked well for me!

My experience making baguettes is that if dough is too sticky it will come out flat. It should be just a little sticky. Kneading has nothing to do with bubbles or flatness. Actually, I think if you only knead enough to reduce the size, that’s the best. Letting it rise for over 2 hours can ruin the taste. That happened once. I think bacteria got in it or something. Often, I’ll punch it down over 8 hours and it is fine. If you don’t have time to bake it, put it in fridge.