Hi ZeldenGM, thanks for the question!
This was indeed a bug that we have fixed and you should be able to enter most lower-level dungeons with up to 10 players, as was possible in Original WoW 1.12. There are some dungeons that had specific caps by the end of Original WoW however. To be specific, all lower level dungeons available through Maraudon should have a player cap of 10, with Blackrock Depths, Scholomance, Stratholme, and Dire Maul being capped at 5 players.
As was the case in Original WoW 1.12, Blackrock Spire should also have a cap of 10 players.