Same issue here. I tried reporting it (FB8951560) but it was marked as "unable to diagnose". Perhaps if you tried to report it as well with the info you have, it might help them...

smbfs support in macOS has gotten worse and worse with every new release. For years, I couldn't reconnect a share using the recent servers menu while Mail was the frontmost app. They fixed it for a brief time, then broke it altogether since 10.15.2, which forced me to use the Cmd K route instead. With Big Sur, mounting a share via cmd k still works for me, but it's become much slower than before. Also, using git on a mounted share is quite dangerous when there are many pending changes: git blocks (probably because the underlying smbfs blocks) and macOS panics when trying to shutdown/reboot the machine. I've always reported these bugs to Apple via Feedback Assistant, but apparently no one cares. Or perhaps I'm the only one who sends them bug reports about samba and they're waiting for someone else to submit reports too, before acting on them... try submitting a feedback report :-)

I've got the same problem, just reported via Feedback Assistant (FB9093021). I suggest you report it as well, since forums might not be read by Apple (in my experience, Feedback Assistant reports are often ignored too, but you have a higher probability of them reaching someone that might fix the bug ;-)