Excerpt from a message I sent to tmux-users mailing list:
I just began to use the recently added zoom feature and came across an anoying thing.
You have panes A and B. A is zoomed, if you switch to pane B, then A is forcibly unzoomed. This is expected, of course, otherwise B would remain invisible.
If you switch back to pane A I expect to get A zoomed again because it wasn’t me who unzoomed it.
See it in action: