Just so I understand you correctly:
Windowed Mode Resolution 1080p >
Game starts in Window >
you press maximize >
Screen maximizes but the fullscreen resolution goes to 4K apparently.
The fullscreen resolution stays at 1080, but that is different from having maximised windowed mode. If I set both to 1080 in the config file it will change the windowed resolution
If I go into settings *after* pressing maximise (staying in windowed mode) it actually just says 800x600, but it is clear from the increase detail/ huge drop in fps that it has jumped to 4k (and verifiable by checking the config file if it isn't made read only).
If I maximise (staying in windowed mode), then change resolution, it makes the window small again.
If I select the fullscreen option it works as intended, but in anything less than 4k resolution the mouse gets scaled up to a ridiculouse size (and I preferred to play in windowed mode for easier swapping between programmes).