A similar issue happens with Apple Silicon macs and external monitors since long time ago. A fix I found online [1] is to disable GPU dithering using the Better Display app.
The flickering was so bad in my case that the pixels got stuck for some minutes when it happened.
I tried that and many other things like changing the default color profile and it and it didn't work. A reboot fixes it for a few days before returning, so it definitely seems like a bug rather than a hardware issue. Both my work MBP (No issues, macos 15) and personal Air (macos26) are apple silicon.
The flickering was so bad in my case that the pixels got stuck for some minutes when it happened.
[1] https://www.reddit.com/r/mac/s/TDOa9Lb5rP