It's embarrassing to admit it was a ground issue. The Raspberry and the Rock Pi are sitting next to each other. The Raspi was powered under a real world scenario via a car battery and all devices were connected to the same ground (minus) terminal. I made a short cut for the Rock Pi, which I powered for this test via 15W wall power supply. Therefore, Rock Pi and devices were connected to different ground and that is bad, very bad indeed. And the Rock Pi even works with the odd 1000 Baud config. Why this is the case I do not understand but I do not care it works.
Vdue got a reaction from atone in Rock Pi 4B+ UART/UART4 no data