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.