Mac Mini M1 display 4K LG27UP850-W why is 4K resolution not working

What cable are you using?

One option is to test a Thunderbolt 3 cable or the expensive Thunderbolt 4 Pro cable.

see > thunderbolt to thunderbolt cables - Apple

The best option for that display, is to use a USB-C to DisplayPort 8K cable for the video.

for example > https://www.amazon.com/dp/B09M38CXZ3/ref=sspa_dk_detail_0?

Then use a separate USB to USB cable for the displays downstream USB ports.

Yes, pitch the LG supplied cable and use a USB-C to DisplayPort cable.

for some example's

see > https://www.amazon.com/dp/B09M38CXZ3/ref=sspa_dk_detail_0?

and > https://www.amazon.com/dp/B0B4D1MS9X/ref=sspa_dk_detail_3?

You're welcome.