It was exactly that. The stupid thing is: I usually shut down my Mac at the end of my workday and on the next day and start everything I need via script and always got funny looks from my co-workers because "you can just close it and keep it running" so I tried it a few days and honestly did not think about restarting because it would have been a fresh start before.
But now I know there's a tmux server running that I can kill when problems occur and I won't need to reboot Everytime tmux starts acting funny. So at least I learned from being dumb and not thinking about basic trouble shooting steps...
Don't you do it? Been using TW for almost two years now and had close to no issues.