Please make sure that you have patch #1 installed. In the RTM version is a bug with reverse incremental jobs to tape.
Did you create the tape job before you had multiple full backups? If yes, try to create a new tape job and choose the reverse incremental job. Now you should get a pop-up which is asking to copy "all" or just the "latest" full backup to the tape. Choose latest and you are happy.
Now my question to the veeam devs: Is it possible to change this behavior? It would be nice if this would be a changeable option in the advanced section. I had to recreate 20 tape jobs because I created them before any full backups were in the repository. Veeam is not asking you if you haven't multiple fulls in the repository. Even it's not possible to remove and re-add the source job. Only delete the jobs and recreate them will help.