I made a new attempt with
Results: Still the same problem, if the right monitor's orientation is portrait flipped.
This time, my test wallpaper better illustratis the problem.
The mouse cursor (left monitor, between the "I" and "J" rows) is only broken when using the inversed color scheme, on left and right display. With Eyefinity off, the inversed cursor fine on all displays. (I forgot to place it on a side display for the photograph, so you'll have to believe me.) The default white cursor works both with Eyefinity on and off, on all displays.
BTW, that odd combination of different cable lengths is only temporary to test with DP-only. If it had helped with this issue or if it will help with my other issue (tearing despite vsync), I would have bought / will buy a third long DP cable and place the PC in the corridor again.
In the meantime I have tested with only normal length DP cables (each monitor with the cable which came with it). Same problem.
BTW, neither did it fix the tearing (tested with Far Cry 5 at 4360x1440, vsync on in game settings, vsync at "on, unless application specifies" in the FC5-specific Radeon settings).
After reading about AMD Eyefinity Pro Configuration Tool (C:\Program Files\AMD\CNext\CNext\EyefinityPro.exe) in Computer Base forum (German), I tried that, but it didn't help with this issue. I still have to set both side monitors to non-flipped portrait.
But that post also explains that you can set the higher refresh rate monitor as primary in Radeon Settings, then set the Eyefinity virtual monitor to high refresh rate (164 Hz in my case). That seems to fix the tearing for me (tested in Far Cry 5 only; in-game settings: refresh rate = 164 Hz, vsync = off, framerate lock = off, anti-aliasing = TAA).
BTW, currently using Radeon Software 20.4.1.
I tried AMD Radeon Eyefinity Pro Configuration Tool again, this time with version 20.11.2. Setting one monitor to portrait flipped mode works now.
(But I don't need Eyefinity anymore, because I ordered a 55 inch TV today intended as my only gaming display.)