Although this behaviour is confusing, I find it quite useful, and often rely on it while weight painting.

If this was changed such that the hidden mesh is also hidden and un-paintable in weight paint mode, it wouldn't break my workflow but I would have to use face masking mode more often, which might need some extra keypresses to enter and leave it, since you can't change bone selection when you're in those masking modes.

For that reason I think this should not be changed unless we also get a faster way to select bones while in a masking mode. It might just sound like a couple extra actions, but already an incredible number of compromises were made at the expense of Weight Paint mode's usability, since 2.80.

Demeter, I understand it is useful when you have some experience with it, and are used to this, but it is very confusing and unintuitive for someone new or even someone experienced but don't use that method much, it took me some time to figure out what was going wrong with it, and I do some skinning too.

I would suggest to keep it, but have some form of visual markings so we understand that those faces are not paintable (greyed out/darkened).

you could change the bones in paint mask modes by selecting them on properties editor> object data > vertex groups, but I understand that is not as easy as picking them on the viewport. and Ideally, as you say, fix the masking options so it works well with picking etc... and we dont have to rely on multiple ways to do the same thing, each of them not great and with different hickups along the way.