Primary monitor glitches when secondary monitor set to portrait orientation

Added a second monitor to my setup this week. Works great if both monitors are in landscape orientation. However, if I set the secondary (older) monitor to portrait orientation, then the primary monitor has some noticeable display issues.

The issues include
- checkerboard pattern is visible when opening / closing windows. this is across a wide area, maybe 2-3 inches vertical and most of the width of the monitor
- trails / glitches when dragging windows - just kinda choppy is the best I can describe it
- checkerboard pattern is also visible when dragging windows, not as large as when opening / closing.

these patterns are fleeting, while windows are moving / opening / closing. if everything is still, no issues.

I just upgraded from 18.04 to 20.04 but the issue persists.

I'm running Ubuntu on an Intel NUC8isBEK unit which has:
• Iris® Plus Graphics 655
• HDMI* 2.0a port with 4K at 60 Hz
• USB Type-C port with DisplayPort* 1.2

The monitors are a new 29" Lenovo and (2560x1440) and an old 24" Samsung (1920x1080)

The DP input on the lenovo is hooked up to the USB-c output on the nuc,
The samsung is plugged into the HDMI port.

When I run xRandr, it appears to show that the hdmi port is empty, and appears to be indicating that i have two display ports setup or daisy chained This is most definitely not the case, as the samsung does not have a DP input, and the lenovo does not support MST, and does not have a DP output.

Here is the xrandr output.

Screen 0: minimum 320 x 200, current 4480 x 1440, maximum 16384 x 16384
DP-1 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 160mm x 90mm
   1920x1080 60.00*+ 59.94 30.00 24.00 29.97 23.98
   1920x1080i 60.00 59.94
   1600x1200 60.00
   1680x1050 59.88
   1280x1024 75.02 60.02
   1440x900 59.90
   1280x960 60.00
   1280x800 59.91
   1152x864 75.00
   1280x720 60.00 59.94
   1024x768 75.03 70.07 60.00
   832x624 74.55
   800x600 72.19 75.00 60.32 56.25
   720x480 60.00 59.94
   640x480 75.00 72.81 66.67 60.00 59.94
   720x400 70.08
DP-2 connected primary 2560x1440+1920+0 (normal left inverted right x axis y axis) 597mm x 336mm
   2560x1440 59.95 + 74.78*
   1920x1200 59.88
   1920x1080 60.00 60.00 50.00 59.94
   1920x1080i 60.00 50.00 59.94
   1680x1050 59.95
   1280x1024 75.02 70.00 60.02
   1440x900 59.89
   1152x864 75.00
   1280x720 60.00 50.00 59.94
   1024x768 75.03 70.07 60.00
   832x624 74.55
   800x600 72.19 75.00 60.32
   720x576 50.00
   720x480 60.00 59.94
   640x480 75.00 72.81 66.67 60.00 59.94
   720x400 70.08
HDMI-1 disconnected (normal left inverted right x axis y axis)

Switching the display settings back to landscape orientation immediately resolves the issue.

So seems like there are two, perhaps related issues / questions.

a) why is xrandr showing HDMI1 as disconnected?
b) any suggestions on how to resolve the display issues with one monitor in portrait orientation?

Thanks
GS