This isn't 100% (I hope and don't think so), but it's very very often that when I use ctrl+z it will undo more than one step. I haven't isolated just how many, but seems only a couple to maybe 3. I also cannot isolate if anything else might be prompting this to happen. I test a lot of things and undo them and redo them quite a bit as I'm working so it's hard to say when or why. I just know it does it a lot and it's a pretty large hindrance to getting things done when I cannot figure out what it might have undone all the time.
There's also a quirk where if you open another page it will not undo from then on. Not 100% of the time also, but quite often.
Would be nice if the undo's would follow on screen so you can see where you are when it stops undoing. This would be helpful even if it didn't have this issue. I know some people like to hit and hold the undo button or key combo, I do it myself every so often as well, and that would be messy if it followed the screen, but maybe have it switch to wherever the last change/undo was made when it stops? Not sure if this is something you want to do or can do, I do know a few of my other editors do this and it helps a lot when you're wiping out a section of things you've just done so you know where you left off.
Ok I've narrowed down a bit of what's happening with this issue. This seems to be "only" when you don't click something outside of what you've changed. For example let's say you are testing some colors or some other types of styles on an element/component on your page. You make a change to the setting and in the App Preview it usually changes immediately and therefore that's where I watch for when I'm testing out settings. Follow these steps and you should see what I mean.
Go into your custom css and edit a color or another style.
DO NOT click anything on your screen at all once you change the chars in the attribute.
Hit the Ctrl+Z to undo what you changed.
It will most likely have undid what you changed just now, PLUS whatever you changed just before that.
This is because in order for your change to actually take place in the app you have to click outside of that property. Usually I just click the attribute above it or somewhere else in the CSS file to activate it. The problem is, I don't always remember to do that and then have edited the last 2 changes (the one I just made and the one previous) and don't realize it's changed something back on me.
Seems that once you edit something if you hit Ctrl+Z it should be undoing what you just did not anything further than that, even if it hasn't been "activated" by clicking something else.
Hopefully that makes sense, but that's my findings on it.