If you like me just pulled drives from your QNAP when expanding instead of going through the “expand, pull one drive, wait for resync, pull another blabla” as the manual states, you can manually expand it by:
replace /dev/md0 with your md device from
mdadm --grow /dev/md0 --size=max
pvdisplay, grab the pv name for the storage pool you want to expand and
tada, all done!