Hey look! I think I found the specific one for you right here!.

Have you tried installing/reinstalling the shiny 4k monitor drivers?

To test you could try lowering the resolution from 'System Preferences > Displays > Display'.

Have you ensured that the expensive dongle for HDMI output isn't incapable of supporting such a high resolution? (And if you happen to have a second Mac other than the shiny new MacBook Pro 13' hanging around for testing, that would narrow it even further!)

You could test with a different display like a TV to see if the issue still occurs.

Have you ensured the expensive dongle for HDMI output isn't defective? You could try booting into - and back out of - Safe Mode. You could try a PRAM reset on the shiny new MacBook Pro 13' (if you haven't already).

After connecting for the first time there is often a firmware update that is necessary for the dongle to function correctly.

Have you checked for software updates from the App Store since the first time you connected the expensive dongle for HDMI output?

Ugreen hdmi glitch