I updated Ubuntu from 16.04 to 18.04 last month, and have noticed some odd behaviour when using TMUX over SSH ever since. (This issue does not occur on the console directly.) To access this shell I'm using OpenSSH within Cygwin, which resides inside a ConsoleZ wrapper. The terminal size is 132x43.
I think what might be going on is TMUX doesn't know the correct height of the terminal window or one of the parent containers. Is this something I need to define somewhere?
Firstly when I open TMUX the only odd thing is that the status bar row is positioned 1 row too high:
If I don't do anything for a while the status bar starts to duplicate:
I can scroll up to see the working directory again, but with the text input position 1 row too low:
If I press CTRL+L to clear the screen the status bar disappears, and I have to scroll down to get it.
Next, if I press CTRL+, to rename the window, each key I type creates a new status bar row:
I've also tested this on a Debian Stretch target, and the issue doesn't occur there, so I'm inclined to think that the issue is specific to my Ubuntu 18.04 VM. Here's the same situation but with a Debian Stretch host: