As far as I'm aware they do know about the problem. I talked to german support via phone (in december) and he said that he already knows about this and that they are already working on a fix. But who knows when and if they will ever fix this.
I was able to fix this issue for my 5120x1440 monitor with Better Display (https://github.com/waydabber/BetterDisplay): I had to hit these buttons under settings and after a reboot I had all the HiDPI resolutions I ever wanted... not in system settings though but in the Better Display menu. But once set you don't need to run Better Display in the background and it looks like it doesn't have any overhead like Better Dummy. Currently working on 4096x1152 which is the equivalent to Windows 125% scaling. Even on a 110dpi screen this looks great and from a performance standpoint runs perfectly well on my 2019 MacBook Pro with two external monitors attached. I really don't get it why apple doesn't enable these options by default.
