Directory became unaccessible after unmount on Ubuntu 20.04

[Perfetto2020]

ifuse ~/Documents/iPhone/
# it works fine
fusermount -u ~/Documents/iPhone

# after 1 or 2 days, i found the directory is unaccesssible
~/Documents$ ls -al
ls: cannot access 'iPhone': Input/output error
total 24
drwxr-xr-x  7 xxxxx xxxxx 4096 Nov 25 21:27 .
drwxr-xr-x 59 xxxxx xxxxx 4096 Dec 19 20:52 ..
drwxrwxr-x  2 xxxxx xxxxx 4096 Aug 21 21:17 books
d?????????  ? ?       ?          ?            ? iPhone