Tape library: HP MSL G3 Series 6.40 (server.domain.com)
Used tapes: TAPE# [TAPE#]
Failed to eject some tapes
Skipping tape media TAPE# eject because lock failed
It never occurs during the week during the (much smaller) Incremental jobs. Looking at the job chain, it appears that it's because the Full tape jobs get queued up and the next job in the queue is taking control of the library.
The tapes show all the .vbk being written to them, and the tape size indicates that the files were fully written.
I'm okay with the tape not being ejected from the drive after a job when there's already another job waiting to write to that tape, but version 9.0 always reported success on the weekend full jobs.