Further update: I have swapped out the cable to my Dell U3219Q display which was a USB 3.1 Gen2 cable for a USB-IF certified Gen1 cable and the monitor's USB hub is now working at full speed. Looks like only certified USB-C 3.1 Gen1 cables work properly.

Have you used system report to see what speed your devices and hubs are connecting at? Some of my devices (which need to connect at USB 3.0 speeds) only connect at USB 2.0 speeds in one of my USB-C ports (and weirdly they don't connect at all in the other USB-C port).

Yep - beta 3 didn't work for me either.