At the moment, a transparency mask in a group with pass-through gets disabled and doesn't affect that group. However, when working with filter layers that would be of great help, since that way multiple filter layers can share one mask. The only solution at the moment is to disable pass-through and add a clone layer into the group, but it's sub-optimal, since you have to group everything below, to make a clone and the filter-layer group can't be dropped to other images without making clone layers by hand.
Interesting feature request, thanks for reporting @Zoom. Yes, it's frustrating about groups with Pass-Trough to can play with the 0% to 100% opacity sliders while not being able to control/distribute this transparency with a transparency mask.
Bumped into this bug and it's super annoying to dance around with complicated nested group setup to use masks and pass-through feature. And frankly don't even understand why that limitation exists at all since you can already change opacity of the group.
Forgot about the version of the krita: 4.0.
*** Bug 399144 has been marked as a duplicate of this bug. ***
The problem is that a pass-through group doesn't have a projection, so the mask would be applied to the entire projection up to and including the group. So, L5 Group Mask L4 L3 L2 L1 L2 would be composed with L1, L3 with the result of that, L4 with the result of that, then the transparency mask would be applied to the result of _that_ and finally L5 on top of everything.