I have a large map image with 21600x10800 resolution.
If I check it in the editor or exported it from code after loading to PNG image, I see it changed to 16384x8192.
It is intentional, if it is, can be switch off?

got it, probably due to 2^x texture size rule

Please remember that the Hardware GPU has limitations for texture sizes.
D.maxTexSize() or something similar named