Finally I get to know what exactly with my first #playdate! As I mentioned long time ago it is one of the first dev-kits. Anyway, there is a trouble with display wire, as I supposed.
Wow, brave! The display on my batch 2 Playdate has started to develop an issue where certain pixels are displaying in a black line. It still plays for now, but I'm saving up my dollars to get it repaired/replaced since it's unfortunately outside the warranty period now.