Thank you for contacting Plugable support! Sorry to hear about this issue. I’d be more than happy to assist you.
Unfortunately this does sound like it could be a macOS issue, we’ve been seeing some severe display issues since macOS Big Sur 11.1 that have not yet been fixed with 11.2.2 or the 11.3 beta: https://www.macrumors.com/2021/02/03/macos-big-sur-external-display-issues
Users are having issues with incorrect resolutions or refresh rates, sometimes unable to get a second display working correctly at all. Since you’re seeing the monitors reported incorrectly as 5K when they’re actually 4K, and because a separate USB-C to DP cable behaves similarly, this does make me think you’re one of the affected users.
10-bit displays shouldn’t have any troubles as long as they’re not HDR or HDR is disabled.
To try and troubleshoot the issue, for the display stuck at 30Hz, which port on the dock is it connected to? “Display 1” or “Display 2”? Does it change behavior if you swap the connections around on the back of the dock? I’d be curious to know if the issue stays with the same port or if it follows the monitor to the other port.
Please don’t hesitate to let us know of other questions.
Thanks again for contacting Plugable support and best wishes!
Senior Engineer | Product Owner