3840 x 1080 scaled not available anymore for M4

[aut0]

I have a really odd problem. I recently upgraded from an MBP with an M2 Pro to an M4. I have an external 5120x1440 49" UW screen.
On the M2 Pro I can get 3840x1080 HiDPI, but on the M4 it selects 3072x864 HiDPI. I tried to add a custom HiDPI resolution in Settings->Display->System Configuration and Flexible Scaling -> Custom scaled resolution, but it does not show up anywhere. Not in "Display Mode" in the BetterDisplay menu nor in the system settings. I tried to reboot, but it still doesn't show up.
Any idea why?

[mojoker]

You ran into the same problem as me, according to apple's official statement, the Mac mini with the m4 chip supports 8k, and my current monitor's native resolution is 51202160, which should logically support 38401440 resolution, but I can only use 3072*1296 hdpi now!

[aut0]

FWIW, I can create a Virtual 32:9 display and then set that to the resolution I want and mirror it to my 49" screen.
That seems to work.
I don't fully understand what I'm doing.
Is there any documentation?

[waydabber]

Currently M4 seems to have 3840px width on 5K2K-s only when the display is 120Hz. This is probably a bug Apple will fix.

https://forums.macrumors.com/threads/5k2k-at-120hz-with-mac-mini-m4.2441289/