M1 iMac only displays 1280x800 via Displayport on Samsung SA850T - why?

On my old Intel iMac I had my Samsung connected with a T3 to Displayport Adapter and it displayed 2560x1440 in 60Hz without problems.

Now I have the new M1 iMac and when I connect my Samsung SA850T I get only 1280x800 at 60Hz.

I tried in the T4 Port of the iMac and the T3 Port of my OWC Thunderbolt4 Dock.

Esult is always that I get only 1280x800. I then tried a USB-C to HDMI Adapter but that is worse because it gives just 1280x720 resolution.

It is the same Thunderbolt to Displayport Adapter that worked perfect on my Intel Mac so I ask WHY isnt it working on the M1 iMac now?