A solution for opened window is forced into maximized
Add a new DWORD registry entry HKLM/System/CurrentControlSet/Control/PriorityControl/ConvertibilityEnabled, and set the value to 0. Save and restart then your problem should be solved.
The root cause of maximized window is the registry key Computer\HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\PriorityControl\ConvertibleSlateMode=0. You can change it to 1 to temporary solve the problem but it will be changed back to 0 after reboot. So the best way is to disable the converterbility function in total.
Both settings were designed to detect and auto setup tablet/convertable mode to allow Windows to have adoptable UI or auto poped keyboard. It is documented here: https://learn.microsoft.com/en-us/windows-hardware/customize/desktop/settings-for-better-tablet-experiences
It seems like recent update Windows 11 start to use this logic to auto set the XBOX experience behaviour, which assumes all window to be maximized. I am not sure how it will go in future but now ConvertibilityEnabled=0 can solve the anonying window size problem.