Preheat and baking temperature

After opening the door the temperature is supposed to drop a few degrees since it's exchanging heat with the outside air (which is usually colder than in the oven...).

The cake batter should also absorb part of the heat put out by the oven, by heat transfer. So if your oven is very small (like an oven-toaster-grill), weak, leaking large amounts of heat (through openings or cracks) or you're putting a huge amount of really cold cake batter the temperature might not rise back to previous levels.

So I'd say it's not normal - at least I never saw such a large temperature drop for such a long time. Trying bake a smaller cake, or checking that the baked goods are not blocking the heating element or the air circulation inside the oven.

Try also putting your thermometer in a different position inside the oven and see what's the difference - maybe just part of the oven is not getting up to temp, since you said your cakes are baking fine.

That doesn't sound right to me, especially as this is a relatively small oven and should come up to heat again pretty quickly (smaller space to heat). I assume here that where you are using this has a reasonable ambient temperature, and you are not using it in a cold place etc.

I would expect a drop in temperature when the door is opened, maybe 10-20 degrees or so depending on the oven capacity, but for the temperature to come back up again quickly, say within 5-10 minutes max. This is the case with my extra-large one piece range oven.

There could be a number of reasons for this:

1. Your measuring thermometer is not accurate. Some of the cheaper analogue rotary versions are not that good, the digital "laser" variety can also give false readings depending on the surface used to measure temperature
2. As mentioned previously, air circulation or blocked heating elements. As the temperature seems more stable without the pan it could be that the pan is preventing the otg's thermostat kicking in, either due to heat being reflected or hot air being trapped
3. The otg's temperature and thermostat settings are optimistic. If this is the case, try setting a different temperature (say 10% higher both with and without the pan) and see what your thermometer says. If the temperature is closer with the higher setting, that could be the issue
4. Different thermal behaviour depending on oven mode. If your oven has different modes, try the same experiment to see which is more accurate. If the oven has multiple elements (and a circular fan type element) one could be more accurate than another. Be careful of using grill mode though, this could easily exceed any in-oven thermometer temperature specifications and damage it.

Having lived with many different ovens over the years, it is a probably a combination of 1-3, with 3 being the most likely culprit if the oven is a cheaper brand. Many an oven I've had to set a bit higher than the thermostat settings to get decent results, but I've yet to come across one where I've had to lower the settings.

Assuming nothing is wrong with the oven (e.g., flaky thermostat), you can try adding thermal mass such as a pizza stone, brick(s), or even pebbles.

Also try making the batter with room temperature milk and eggs.