Yep. At one place I worked, we did a big off-site disaster recovery exercise every year.
Most of the time it went fine, but there were multiple years where a restore didn’t work due to an issue with one or more tapes. Either the data and/or indexes couldn’t be read, or the tape physically failed during the restore.
Backups aren’t backups unless they’re tested.





Caveat: I’ve used LUKS quite a bit, but never to do exactly what you’re trying to do, so the following is a guess.
If the root disk was the encrypted one, the tooling would add scripts to your initramfs to handle the prompting and unlocking at boot.
Since it’s not the root, those scripts may not be getting added.
I’m not sure about the “right” way to force it to add them, but you should be able to manually tweak your initramfs setup to add them.