I have the exact same issue. It happens all the times on Mac OS (Big Sur 11.6.1) when both Slack and Microsoft Remote Desktop are running. First is Slack to freeze, second is Microsoft RD.
I could partially reproduce it on Windows 10 when I'm connected over VPN, Slack freezes for a minutes or so, then it becomes responsive again.
On Mac OS once it freezes I need to forcibly close both Slack and Microsoft RD.