enlarging pane in tmux (C-b z) kills a pytest run

Is there a reason for my local pytest run to die right after I Ctrl-b z? Is there a signal that is sent and not catched by the tmux server and thus, somehow killing my pytest? (I think running the pytest in a non-tmux terminal, Ctrl-b z won’t do a thing though I couldn’t confirm that yet)

The same happens when trying to resize a pane (i.e. Ctrl-b Press-Down-Arrow)