`devmon --mount-all` always returns exit code 3

[lukehutch]

devmon --mount-all and devmon --mount both always return exit code 3, whether they succeeds or not:

pi@raspberrypi:~ $ sudo -u pi devmon --mount-all
devmon: /usr/bin/udevil --mount /dev/sda --mount-options "noexec,nosuid,nodev,noatime"    # (TuxPE10.2)
mount: /media/pi/TuxPE10.2: WARNING: device write-protected, mounted read-only.
Mounted /dev/sda at /media/pi/TuxPE10.2
devmon: /usr/bin/udevil --mount /dev/sdb1 --mount-options "noexec,nosuid,nodev,noatime"    # (FREEDOS)
Mounted /dev/sdb1 at /media/pi/FREEDOS
pi@raspberrypi:~ $ echo $?
3
pi@raspberrypi:~ $ sudo -u pi devmon --mount-all
pi@raspberrypi:~ $ echo $?
3

I would expect exit code 0 unless something goes wrong.