I'm not using the laptop panel or connector. This is a Thunderbolt eGPU.

HDMI is direct into the external RX 560.

While testing I just realized this behavior has nothing to do with multi GPU.
Ran the same glxgears test on desktop that only has RX 480 and iGPU disabled in
BIOS. Same behavior.

$ DRI_PRIME=1 glxgears
Running synchronized to the vertical refresh.  The framerate should be
approximately the same as the monitor refresh rate.
28663 frames in 5.0 seconds = 5732.486 FPS
26437 frames in 5.0 seconds = 5286.734 FPS
26203 frames in 5.0 seconds = 5239.404 FPS
26759 frames in 5.0 seconds = 5346.603 FPS

Will attach the current Xorg log.