Do you have a pet? A cat may take a liking to your plant. Just a conjecture on my part. Looks like physical damage to me. Also it maybe useful to post a pic of the whole plant to see the overall health of the plant.

When did this start? The bottom picture with bark damage parallel to trunk/stems looks like perhaps gummosis, caused by phytophthora. The upper picture looks like damage is against the grain more from abrasion, but could be you picking at the dead bark.

I'll try to take a picture of the full tree later. Last week is the first I've noticed the bark peeling. It seemed very minor and the tree overall looked healthy, so I didn't think anything of it. I went downstairs today to water it and noticed the leaves were a bit droopy, so I took a closer look and noticed that the damage was more extensive on the other side when I rotated the container.

I don't have any pets and I can tell you I haven't picked at it or even touched it all. In fact, the trees have been in my basement under a grow lamp since November, so no animals have been able to get at it since that time.

The only other thing I can think of worth mentioning is that I have been fighting off occasional gnat-like flying insects over the last month. I know from previous experience that where there is one, there are usually a bunch waiting to hatch. Insecticidal soap and oil sprays never did the job for me in the past, so I just go straight for the Sevin these days. I seem to be keeping them down and I've used it on my lemon tree before in previous years with no issues. I don't think there is any relationship between the two though so I neglected to mention it before.

"gnat-like flying insects"

Could those be fungus gnats? Look them up online; if so, they're fairly harmless and relatively easy to get rid of; no Sevin needed. That's what they sound like and the most common thing you'll see that flies around soil-bound plants in winter indoors.

If so, two ways to get rid of them:

At the surface, either put out some fly paper (if you can find it) or a little glass of apple cider vinegar to catch the adults.

On the soil, you can crumble a BT mosquito dip and water it in (it'll kill the larvae), or dilute 1 part hydrogen peroxide from the drugstore (3%) with 5 parts water. Water the plant such that the mixture reaches the roots the fungi are feeding on (they're light feeders). They'll die on contact.

Any combination of above/below ground treatments works great on fungus gnats.

I agree with Tropic. the bark damage certainly is not from fungus gnats.

Yes, I suppose "they're fairly harmless" might not have been definitive enough.