I bought a cheap, generic Tuwejia adapter from Amazon.. it has USB/HDMI/USB-C going to USB-C. I am very pleased with it, it does a great job!
My big Dell keyboard works well and the big trackball mouse is attached via the keyboard, not the adapter, and both work very well. The USB-C charges the phone very well. The HDMI part.. I do not believe it's the adapter's fault, it's my cheap 2K monitor. Okay.. here it goes..
I had another generic adapter similar in size and shape, but it was plastic. The phone never charged on that one. The Tuwejia feels solid and the cover is metal, aluminum? The phone charges very well on that one.
I am using a cheap QNIX monitor (QX2710) and it seems my phone's showing 640 x 480 (I can't tell but it's really bad) on my big 27" 2K monitor.
I had the same problem with my LG V20 so I would take it to the big 1080p TV in the living room and the phone would somehow "reset" itself to 1080p instead of lower resolution.
I tried it again with the P20 Pro and it displayed 1080p in the living room but when I brought it back to my office, it was still very low resolution. It doesn't seem the phone resets itself after being hooked up to a 1080p monitor.
Again, I don't think it's the adapter at all. It's the monitor, I'm almost sure of this.
EDITED TO ADD: I just went to Best Buy and got a Samsung 4K monitor.. I am happy to report it's now much better so it was the cheap monitor. I have the adapter and have been using it like a desktop for the past hour. It's still showing 100% charge. I have my USB keyboard and mouse attached. It's wonderful!