Hi Stephan. As I can see you either disabled a barcode reader in the library or are using tapes without barcodes. Could you please explain the reason for that? If your tapes had readable barcodes, such issue wouldn't have appeared.
Actually the logic of this behaviour is common: Veeam B&R is unable to inventory the tape for whatever reason but counts that the tape in slot didn't change. We will think about improving the logic in that particular case. As a workaround (apart from labeling tapes with barcodes) I can suggest that you first remove written tapes from the magazine, perform library rescan, written tapes should go offline, and insert new tapes afterwards. The library will recognize them as new ones.