Hi there! 
System info:
Raspberry Pi 3B+
OMV v4.1.7 Arrakis
When setting up an external thumb drive with an exfat (fat32) filesystem as a SMB share the default setup produces permission issues and the mounted share isn't writable.
When mounted for the first time from the Web UI the system mounts the partition with the following options which work
rw,relatime,fmask=0000,dmask=0000,allow_utime=0022,codepage=437,iocharset=ascii,shortname=mixed,errors=remount-ro
However, following a reboot, write acces is lost, and the line in /etc/fstab doesn not specify dmask and fmask options that get mounted as 0022 and stop working.
The problem is fixed with manually editing /etc/fstab and adding options to drive like so
vfat rw,fmask=000,dmask=000,nofail 0 2
I'm not sure why this happens but thought it might come in handy and could help fix something in a future version!