I have tested a few cards myself and work fine with or withouth hardware acceleration depending on the model of the card. For now i am using the RX570 in PCIeX16 slot for AmigaOS and X1950 as second GPU for Linux / MorphOS use.
I do have a monitor connected on each GPU, and running 3.17 with the latest fix from Bigfoot.
You could try (if you have one) another card to see if the problem exists..
