The GPU may not be going into it's lowest power state at idle. That could be a driver limitation, due to load from the desktop environment, or because the wrong driver is loaded.On my secondary Linux PC, my AMD RX 550 GPU fan isn't stopping when my PC is idle in Linux Mint. However it does if I boot into Windows 7. Does Anyone know how to fix this issue?
Oh, also, what is the monitor refresh rate and resolution? If it's high (4k, >60hz, for example) the card might have to enter a higher power state even with low graphical detail and effects in the desktop environment.
Edit: For options to control the fan speed, regardless of the reason it is higher, see this wiki page: https://wiki.archlinux.org/title/fan_speed_control#AMDGPU_sysfs_fan_control
Dunno if those services are available on mint, but should get you pointed in the right direction, anyway.