I often have multiple montages open, and switch between them. I also usually expand or contract the montage time axis by dragging up or down from the time ruler.
If I switch from one montage tab to another and then immediately drag upwards from the ruler, then as the cursor leaves the ruler it changes, and the expansion stops happening (going downwards is not a problem, but reversing the mouse and going above the ruler still has the problem). Subsequent drags are correct.
9.5.15 on Windows
PS - this also happens in other circumstances, but I haven’t been able to reproduce them.