Probably need to find it and delete it. Assuming you wish to control it all with the dropdown, make sure that About → Settings and the job’s screen 5 Advanced options have no –retention-policy or –keep-versions or
–keep-time, and (in theory) the dropdown should not allow you to set more than one of its choices at once.
You should be able to see the merging of the global settings and job options using Export as Commandline or Commandline (or run the backup and see if it complains). I think Export to File shows only job’s options.
You could try removing every one of those three options you can see anywhere (remove dropdown option by choosing “Keep all backups” from the dropdown), then see if somehow there’s still one of the three left.
Once everything is gone, try the dropdown again. If need be, we can look in databases, but let’s try UI first.