Are bubbling preserved lemons a sign of spoilage?

This is totally not a problem.

This style of lemon preservation relies on fermentation. The salt is not intended to halt all fermentation- it just restricts it to the tasty kind. Fermented pickles are a common and traditional way to preserve food because the salt and acid and thriving tasty bacteria make a very inhospitable environment for bad bugs.

The production of gas is just a symptom of the fermentation and not a problem. I regularly make Indian lemon pickle, which sounds like a similar recipe, and it produces some amount of gas. Recipes will typically call for the lid to be gently closed to allow some of the gas to escape- just to prevent the bottle from exploding.

This is not your typical canning. Typical canning uses acid or high temps to stop all bacterial growth. In that kind of canning gas production would be a very bad sign and you would have to throw it out.

It's probably fermentation, and as that requires microbial activity it's a sign your food has not been preserved. Unless the recipe specifically says this is expected and desired I would not eat it.

The recipe you describe is a "wild fermentation" recipe.

So, as @Sobachatina said, the bubbles are a good sign, not a bad sign.

The strategy with "wild fermentation" is to create an environment that gives "good microbes" an edge over "bad" ones, in such a way that their advantage continues to increase over time. Salt gives the good ones an initial edge--most bad microbes are halophobic. Beyond that, most good microbes digest carbohydrates and produce CO2 plus acids and/or alcohols, and most good microbes tolerate acidic environments and up to moderate amounts of alcohol better than the bad microbes. At some point, the baddies have zero chance of making any progress.

Canning is a very different strategy: create a hermetic seal, then kill them all, good and bad, with heat. The challenge is that if you fail to seal it tight, or if you fail to kill them all, then what you have is a tabla rasa for whatever microbes wind up in there. And some of the microbes that enjoy a mid-pH high-moisture anaerobic environment can cause serious problems, like botulism!

So for fermented foods, bubbly = good. For canned foods, bubbly = bad.

PS I would discourage you from putting a float of oil on top of any ferment. Little bits of stuff suspended in oil can create just the sort of environment that botulism microbes enjoy. Lemons are acidic enough that it may not be a problem in this case, but it's a bad practice.

You didn't "can" anything this time, either. You made "SaeurLemon" ( a salt-fermented "cure" like sauerkraut, only with lemons, not cabbage) - Gas is perfectly normal.

If you HAD "canned" it, bubbles would be bad. There, you're sterilizing (or trying to) the food with high heat and vacuum sealing it. It should remain pretty much as canned, and there should be a vacuum seal until you open the jar. Anything making gas in "canned" food is highly suspect. That is not what you did...

This sounds to me like a wild fermentation. Yeast will happily consume the sugars in your jar and create carbon dioxide (the gas you noted) and perhaps some ethanol and various flavor esters. This in itself isn't necessarily a problem, but where yeast can thrive so can many other organisms. The key to avoiding this in the future is to make sure everything that touches the ingredients is sanitized (the jar, spoons, juicer, etc). What you have many not be too bad (depends on what else is growing in there) but it probably won't taste good either. I'd toss it and try again.