TLDR; Try disabling Time Machine or removing faulty Time Machine targets. After working on this for awhile, I think I've figure out what's causing the issue that I'm seeing with umount and Safari. Each time that umount is stuck and Safari is non-responsive, Time Machine was failing to access a disk over the network. I've seen it both with a Synology Time Machine share and a Mac OS Server Time Machine share. Removing the faulty Time Machine target or disabling Time Machine altogether appears to result in the issue no longer happening. I hope that this helps at least some of you.

I'm having this problem too on several Macs, but sporadically. I've noticed that when it occurs, that umount appears in the Activity Monitor spiked at 100% cpu. I am unable to kill the process even with force quit or kill -9. After the process eventually crashes, safari goes back to normal. If anyone else is still experiencing this issue, can you check your activity monitor to see if umount is running?

I'm having this problem too on several Macs, but sporadically. I've noticed that when it occurs, that umount appears in the Activity Monitor spiked at 100% cpu. I am unable to kill the process even with force quit or kill -9. After the process eventually crashes, safari goes back to normal. If anyone else is still experiencing this issue, can you check your activity monitor to see if umount is running?