after more testing, it seems the problem is due to the HDMI port.
I have a dual display setup, the main EIZO monitor is going through an OWC hub and a DisplayPort cable.
the secondary monitor however, is connected to the native HDMI port of the Mac Studio.
As soon as I login, the main monitor will show a reduced (non native) resolution.
But if I unplug the second monitor temporarily, the resolution on the main one is instantly changed to the native one.
I reconnect again the 2nd monitor, and boom, low resolution in the main one kicks in.
After I login, everything is fine, except if the monitors go in energy saving mode.
At that point, when I move the mouse again to wake them up, the resolution on the main monitor is messed up again.
And I have to manually disconnect the HDMI again, with the following mess of windows moving around, screwed layouts, and so on.
I also spoke with a Genius in an Apple Store and he had no clue except "try to post on discussions".
My suspect is that the latest 12.3.1 update changed something related to HDMI and displays in general (as it solved some previous display problems, it created new ones).
This is very frustrating because there is nothing I can do except manually unplugging the cables each time.
I'm reading on the web similar problems, I really hope there's something that we can do in the meantime.