When running in tmux it looks like I get some content leaking into the terminal

It seems to happen irregularly; when pressing and holding a key, or sometimes when moving and drawing in a new cell.
This may be related to the workings of tmux, but it would be ideal to run safely by default.
Reproduction (MacBook Pro in Terminal.app):
pxltrm then move and draw (works)
tmux; pxltrm, then move and draw (does not work)
When running in tmux it looks like I get some content leaking into the terminal
It seems to happen irregularly; when pressing and holding a key, or sometimes when moving and drawing in a new cell.
This may be related to the workings of tmux, but it would be ideal to run safely by default.
Reproduction (MacBook Pro in Terminal.app):
pxltrmthen move and draw (works)tmux; pxltrm, then move and draw (does not work)