Well if it's using 3x scaling, then writing one pixel will write a 3x3 block on a 1242x2208 buffer that gets downscaled to 1080x1920 yielding a result like this https://imgur.com/ZsJKxzV

Hmm, what if the checkerboard pattern is at 1:1 (1x1 blocks)?

Depends on which downscaling algorithm(s) they use

Moiré patterns probably.