Could be oedema, try watering less often, wait for the soil to almost completely dry before watering again.

Do I see dozens of tiny pin prick marks on the back of that leaf? Mites. Those little pock marks on the underside of the leaves are a strong indication.

Not all mites spin webbing, and some are microscopic. I got an infestation in one pot that almost wiped the entire pot out within just a few days of me noticing the first damage. I wouldn't wait. Just treat the plant. It's better to treat an infection it doesn't have, than not to treat mites and lose the plant. They're nasty.

I had great success with Bayer Advanced 3-in-1 for flowers and shrubs. It has a mitacide, an insecticide, and something for fungus, I think. I played with home made insecticidal soaps, which worked initially, but then the mites came back.

Mites tend to like dry air, stressed plants, and fertilizer. Don't over-fertilize. You say you've kept the soil damp. Most plants prefer the soil a bit drier than most people think it should be. Let the top 1/4 to 1/3 of the soil dry out between waterings. Soil that is too damp will cause leaf drop like you describe, so if those aren't pin pricks on the leaves and you don't have mites, too much water could be the problem. I think you have mites though.

addendum:

in severe infestations, the pin pricks will appear on the front of the leaves also, and leaves will become distorted at the margins. It's especially noticeable in new leaves. Lots of little pinpricks equal one brown spot.

You need a product that says on the label that it'll kill mites. Insecticides won't work on mites.

I just have my small phone screen to view the pictures, but that sort of wavy edge can be caused by mites. I got rid of them on my Schefflera with homemade insecticidal soap, but when the new leaves started coming in, they were dimpled and distorted in places. The mites were still there, but there were fewer of them so they didn't completely kill the plant.

Schefflera leaves are pretty thin. I can't tell what your tree is, but the leaves look thicker than. A Schefflera, so I'm not sure the damage would be identical. Of the half dozen or so of my plants that got the infection, it looked different in 4 of them. I think it affects different kinds of leaves differently.

If you hold a clean piece of white paper below a leaf and flick the leaf sharply, you might dislodge some mites onto the paper. Watch the tiny motes of dust on the paper for a few seconds, and they might begin to move. If they do, maybe you can see if they're mite-shaped. If you don't see anything, that doesn't mean you don't have mites. I couldn't see mine. I once got a macro photograph of one, and photos of eggs in a particularly tough plant that survived the infection for months untreated (because I hadn't figured out what it was). I wouldn't suggest letting your plant go that long, it was a bizarrely tough plant, and 1 of 2 of them eventually succumbed.

Tiny indentations on the leaves like pin pricks are telltale. The mites suck plant juices and leave little indentations everywhere they feed. I've read that they prefer undersides of leaves, but they worked both sides of mine.

No - the pin pricks are visible in the top right picture - the one in your hand. Looks as if something has been grazing on the leaf in very tiny little nibbles.

Do you know what the plant is? Is it actually an indoor plant? And it is really Bonsai or just a compact plant?