Umount restic backup share - target busy

I get the following mail 14.03.2021, 23:46:

umount: /mnt/backup-neth_restic: target is busy.
(In some cases useful info about processes that use
the device is found by lsof(8) or fuser(1))

But checking the backup, it apparently was successfull. At least it seems like:

But why is the unmount not done? It should unmount the share after the backup, shouldn’t it? Is this a bug? Could it be that because of the size of the backup the unmount can’t take place?

What additional info could I serve to find the source of this problem and fix it?

Unmounting the share manually this morning worked. After unmount ls -l /mnt/share shows that there are no files or folders stored within the mountpoint.