I think it was with Groom, but it may have been another gainer. I was gaining Night Watchmen to my discard pile.
I just tested Groom, and it gained Night Watchman to my hand.
If it's a card like Develop or the Event Demand that specifically says to gain a card to another place (Develop, Demand: on top of your deck), that will take precednece over Night Watchman.
If you see this behaviour again, note down the game number and turn. That makes it easier to see what happened and if there is a reason for the behaviour or it is really a bug.