I’ve seen a few posts about this. There is indeed black smearing of OLED decks. Use the following link below and set to a lower brightness while you are in a dark room. You’ll be able to see a purpleish tint following the square. Many OLED screens have this. It’s not just a Deck issue.

EDIT: This seems to only be the Samsung panel and not BOE.

https://twitter.com/marcedwards/status/1053519077958803456?lang=en