Using DietPi-Drive_Manager to mount the drive, there are no errors and fstab already shows mounted, after restarting the computer the drive is still unmounted.
This is just a configuration file and is not given an information if the drive is mounted or not. For this you would need to use mount command.
The drive is configured to be mounted as soon as someone tries to access the folder. It is not done automatically on boot. Try following to see if the mound works as expected
Yes this is correct and proven the automatic mount working. Same happen if other apps or processes tying to access the drive as it will trigger the same automatic mount. Therefore it’s is not needed to see the drive right after reboot. There is no benefit of having the mount available without being needed. More the other way around, as it could be slowing down the boot process if the system is waiting for the drive to become available.