binary in shared folder loses executable permission while running

[nijhof]

After upgrading from virtualbox v5.0 to v6.0.10 I was not able to execute a binary from a shared folder (host Windows, guest Ubuntu) twice in parallel: while executing its permission would change from -rwxrwxrwx to -rw-rw-rw-.

I had mounted it via a line in /etc/fstab:

FolderName  /home/somewhere vboxsf rw,uid=1000,gid=1000,dmode=755  0  0

how to reproduce: with that mount:

cp /bin/cat /home/somewhere/cat2
ls -l /home/somewhere/cat2 # => permissions -rwxrwxrwx, owner me:me
/home/somewhere/cat2 &
ls -l /home/somewhere/cat2 # => permissions -rw-rw-rw-
/home/somewhere/cat2   # => 'permission denied'

After stopping the running executable the permissions return to -rwxrwxrwx again.

Mounting as an automount instead resolves the problem: cat2 has owner root:vboxsf, and permissions -rwxrwx--- - and that doesn't change while running.