Very odd indeed.
This is typical of X-Plane’s Vulkan engine, particularly on the ground, in my experience.
Can you provide the detail requested here please? And also a recent Log.txt file? Thanks.
Also have you tried the following?
P.S. It’s a 60 Hz 4K panel but only drivable at 60 Hz via DisplayPort. That explains the “30 Hz” question. Not that unusual after all, due to HDMI limitations of past.