I have seen this issue since pre-patch. I had several post-50 alts that i couldn’t level and run dungeons for the LFG items i needed for All-the-Things. I thought i posted back then but it didn’t get noticed.
Edit to link previous thread
https://us.forums.blizzard.com/en/wow/t/cant-queue-for-cataclysm-dungeons-post-level-50/1403726/8