I also had problems with my Lg 5k2k ultra wide monitor. Thanks to a user in another post, I was able to solve. I don't know why but in my display I had to change the displayport port from v1.4 to v1.2. Even though I don't use that port but the usbc port (this display has a usbc/thunderbolt port), I had to make this change. Once done the display was recognized well. So if you have monitors with many type of ports, I recommend that you see your monitor settings and play around with the settings a bit.
Another advice if you use the hdmi port, is to check if you have an option called “hdmi ultra hd deep color” and activate it. It is called so in my Lg 4K monitor (this is a 16:9 and not ultrawide, yes I have 2 external display). If you have another brand, try google for search a similar option, maybe it’s called in another way. In my case I had not activated this option and therefore the Mac mini M1 saw it only at 30hz or at 60hz but not at native resolution (everything was blurred).