I had this problem with our Dell 124T until I checked the 'Eject media once the job finishes' box in backup to tape options. From doing some research this option really means 'put the tape back in the autoloader slot' not actually eject the tape to the mailslot.
Also, is there any way to force unlock of the autoloader from B&R 7?
I've also got some strange locking happening when I'm changing the tapes. When I tell the autoloader to eject the tape it tells me "system locked" but a retry of the eject from the same slot works fine. Until the next slot, which tells me "system locked" again. I just assumed it was Veeam B&R detecting the media change and using the barcode scanner to work out which tape was where