Extended displays with wrong proportions

Everything works as expected.

Your screens have different physical pixel sizes.

• Dell UP3214Q's pixels are 0.182 mm in size with density of 140 PPI (pixels per inch)
• Dell P2213's pixels are 0.282 mm in size with density of 90 PPI

Task Manager window on your screenshot is 806px high. Divide that by screen's density and you get respectively 5.76" (146 mm) and 8.96" (227 mm).

From what I see on your screenshot, scaling works properly: larger screen has higher density, so Windows upscales everything to make it reasonably readable and crisp. Stuff on smaller screen uses default scaling of 100%.

When a window is split between two screens, Windows has to choose whether it will scale it or not. It can't do both - window needs to have fixed dimensions. It can't be 806px high on right half and, say, 537px on left half. Once the window is entirely on smaller screen, Windows will stop scaling it.