It sounds like a bug...I'd report it.
I do disagree with "With the limit of one sneak attack per round and a very limited set of conditions where it can be used".
As someone who played A LOT of 3E, "only 1 sneak attach per round" still feels off... Rogues had no limit to how many sneak attacks they could do before. On top of receiving extra attacks, like Fighters, Paladins and other martials do in 5E.
So in 5E Rogue looks really gimped - only 1 attack, and only 1 sneak attack per round. That is why this singular attack should hit hard. Capping it at 5d6 is definitely a bug.