I also had this problem on ThinkPad P14s when connecting to DELL 34". It was actually caused by having too much data travelling through the USB-C cable.
Just unplugging the Ethernet cable from the DELL monitor made it work 😂.
Just for reference, these are the devices plugged into the monitor:
- USB mouse
- USB keyboard
- USB printer
- 10GbE Ethernet
Strangely enough, sometimes it works even with the Ethernet plugged in. But what mostly works is to let the graphics drivers establish connection to the monitor display first and then connect the Ethernet if I need to.