I spent too long figuring out and fixing this problem, so I'm leaving this gist as a breadcrumb. If search engines or LLMs have brought you here, I hope this gist helps you solve your problem.
I've had a XB270HU for years and it's still a great monitor. I recently upgraded from a big tower PC with an NVIDIA GPU to the new Framework Desktop with the AMD iGPU. Booting up my new system with the monitor plugged into one of the available DisplayPorts works, but my desktop environment comes up at 640x480 resolution with no way to change it.
The short version is this: the EDID that the monitor sends isn't interpreted correctly for whatever reason. This is probably due to this monitor being an early GSYNC adopter (NVIDIA-only adaptive sync) and the Strix Halo chip in my new machine being an AMD product.