To reproduce the bug (Mac OS Big Sur 11.6.1, but also happened on Catalina): 1. Load Slack 2. Use Microsoft Remote Desktop by remoting into another machine. Make sure "Clipboard" is enabled. ...
I've consistently had this issue for well over a year. Big Sur, RD 10.7.4
Most often happens when I Cmd-Tab from my RD session directly to Slack, but this isn't always the trigger. Quitting RD frees up Slack again. The opposite is not true.