Amanda, I don't know the answer precisely with regard to Meyer lemons, but I know that, with lacto-fermented vegetables or homemade sauerkraut, a couple of bubbles at the surface is OK, but you don't want excessive frothing, a fermented (fizzy) taste, or thickening of the brine. I'd probably be cautious with your batch of lemons.

I have given up on preserved lemons after one batch got black mould, and another had a LOT of white Ickies all through it....husband refused to even think about eating it!

I've been making batches of preserved Meyer lemons for the past 3 years , never seen it fermenting. , my latest batch from 3 weeks back was cut up into small pieces before salting and even with that level of exposed interiors, no bubbles.

Hmmm, interesting.

What's your preservation method? Salt and juice? Salt and water? Something else?

A small checklist to go through:
Is there iodine in your salt?
Do you use antibacterial soaps/detergents/anything in your home?
Are your lemons organic? Are they imported?

Iodine and antibacterial soaps are often the cause of moldy ferments as they kill off the beneficial invisible beasties.

As for lemons fermenting on you... I wish I had more experience preserving lemons. Going from general theory, perhaps these lemons have a different acid content than other kinds of lemons, so will react differently and/or may require more salt than the other kind.

Going to keep a close eye on this thread as I've just started preserving my own lemons.

I really love questions like this. Fermenting is so tricky - wild indeed! I had a wee mince explosion this year, but in the end all was delicious.

I agree that one's preserving methods, and the "ambient" temperature of their home, has a lot to do with what occurs, and what can occur. After about 5 days at room temp., I put my lemons in the walk-in (usually much colder, and more stable, than a home fridge) with a full covering of lemon juice, and then olive oil, for extra "protection." I haven't noticed any fermented taste, or action... I used Meyer Lemons too.

Do let us know what you've discovered? I LOVED learning about how iodized salt kills good microbes. I'm a huge proponent for Kosher Salt myself, preferring it's "warm" taste, over the bitterness in "table salt," or the high acid of sea salt.