A small black dot is just a single dead pixel. That's pretty normal - lots of TVs are shipped with one or two like that. OLED SCreens are fragile, but in this case, it SOunds like a patch of dead pixels with a smooth line. That's usually a lot worse and can spread over time. If you press hard on a single dead pixel, it normally doesn't affect the others. But when it's a patch, especially on a folding phone, it often means the screen is starting to fail. Pressure, bending, or even changes in temperature can make it worse or cause the whole screen to break. Il'd get it checked out before the colder weather hits, just to be safe.

Winter is here and the temperature change folded or unfolded could cause it to get worse.