Until update to:
[greg@greg-pc ~]$ uname -a
Linux greg-pc 5.4.67-1-MANJARO #1 SMP PREEMPT Wed Sep 23 14:20:18 UTC 2020 x86_64 GNU/Linux
[greg@greg-pc ~]$ lsb_release -a
LSB Version: n/a
Distributor ID: ManjaroLinux
Description: Manjaro Linux
Manjaro was one of two distros (other is Mint Linux) that support automatic multi-seat creation when a Plugable UD-160-A USB connected zero client is hot plugged. Since this update, the seat is created, but no session is assigned to it so the seat displays an active monitor, but no login greeter, so the screen is blank. You can copy data to its framebuffer which will then show corresponding pixels. Thus the udldrmfb driver is working correctly, but no session is getting connected to it.
The integrated main computer display works normally, but the Display app does not see this USB display (commented out line in /usr/lib/udev/71-seat.rules for this test so UD-160-A would be part of seat0 instead of a new seat).
For more detailed discussion of multi-seat computing, see my
multi-seat dot com