Now I just want to interject.
Critical Role,. as I've conceded, is entertainment. First and foremost. Matt gets a pass when he makes decisions that fit his narrative, because he's producing content to entertain. His players aren't just players playing a game, they are actors, and are, presumably, complicit in the narrative.
So when I talk about this incident, I'm talking about how I would respond to it in a real, actual game of D&D.
Now.
You're the DM of a group of players. You've approved the characters (and possibly have DM'd for them for more than one session). How on Earth could you have a problem with a core Feat of one character, which they presumably use often, that you know exists, and how it interacts with a scenario you created?
This isn't like someone swapping their spell list out today! Surely in your mind you could have said "Gee, I wonder how that Sentinel Feat is going to affect my encounter?" well in advance?