There’s some component in your docking setup (the dock or the TV) that’s causing this. I regularly dock to a 1440p monitor and a 1080p projector with zero issues.

That being said, there are multiple docks out there (even the official one) that don’t jibe with both the Deck and every display and thus give a subpar experience, as there are widespread reports of that. You kind of have to luck into the compatibility of both of those components, and even then there are also TVs out there that aren’t really dock-happy with anything.

Try docking to someone else’s TV next time you go visit someone, if it’s fine then you know it’s either your TV or your TV and dock not liking each other.