The procstat output shows that lklfuse is stuck in unmount while waiting for a
reponse from the server (i.e. itself). That suggests that there's an ordering
bug in the shutdown procedure. It's as if the FUSE thread shuts down before
the unmount is complete.
This bug is also reproducible on 11.3-RELEASE-p1, so the bug lies in lklfuse,
and not the new fusefs(5) driver.