I use Arch on a Thinkpad T480 with the Lenovo Thunderbold 3 dock.
Hi, I have the same dock and I have had loads of problems with it. Almost all of them resolve and reoccur somewhat randomly - nothing seems reproducible. I think the issue (for me) is the dock, not the laptop.
I don't know why this doesn't work anymore. Could it be a driver issue?
Do the displays work correctly again with a cable other than DP, like HDMI?
You could also look into your BIOS firmware - there are usually a few updates available on Lenovo's website (bootable Windows ISO) and if you're lucky they'll be available via fwupdmgr from Linux. There might be firmware updates for the actual dock, too. I'm laying low because my dock works *most* of the time.