activities-revert from within a window in a modified activity frame (different buffers showing) reverts only some of the windows. When called from a window containing a correct buffer from the activity, it works as expected to restore other windows.

For example a given activity contains 3 window/buffer combos, A, B, and C. If A and B are altered to show other non-activity buffers (e.g. with winner), to say an A', B', C, calling activities-resume from A' might lead to a spurious C, B', C configuration.

I think I have identified the reason for this. If one of the files (say A's) has gone missing, due to changing git branches, for example, on revert its window will reappear, but the buffer contents of the window will be arbitrary, sometimes selecting a file from the activity, or another file altogether.

Here perhaps a warning would be appropriate, if attempting to restore a file-backed buffer which has gone missing (and also not creating the associated window).