So Thunderbolt isn't required, only USB 3.1 Alternate Mode and a powerful (probably only *integrated*) GPU that can link to it.
You might have hit some kind of bandwidth limitation using both USB-C ports at once. Although that doesn't make sense at Thunderbolt 3 speeds, it would make sense at USB speeds. And even if USB 3.1 speeds are being surpassed to support Alternate Mode your chipset/GPU might only know how to do it on one port at a time. So this might not be fixable...
Also remember to try 'Ubuntu on Wayland' which uses a completely different code path for all this.
Thanks for pointing that out. I was wondering why so many adapters/cables for sale only mention "USB" while also claiming to do 4K60Hz...
https:/ /www.displaypor t.org/displaypo rt-over- usb-c/
So Thunderbolt isn't required, only USB 3.1 Alternate Mode and a powerful (probably only *integrated*) GPU that can link to it.
You might have hit some kind of bandwidth limitation using both USB-C ports at once. Although that doesn't make sense at Thunderbolt 3 speeds, it would make sense at USB speeds. And even if USB 3.1 speeds are being surpassed to support Alternate Mode your chipset/GPU might only know how to do it on one port at a time. So this might not be fixable...
Also remember to try 'Ubuntu on Wayland' which uses a completely different code path for all this.