This is the system:
Sapphire PULSE RX 590 8GB or XFX RX 570 RS 8GB (1st x16 slot on the board)
FX-8350 @ 4.6GHz
2x8GB DDR3 1866 RAM
PCIe x4 M.2 SSD (in M.2 header)
Rosewill H.I.V.E. 750W PSU
No other board slots are being used.
All temps are normal.
I Tried both my RX 590 and RX 570 (individually), and they both remain at PCIe 1.1 while running Furmark and GPU-Z Render Test.* Yet, two of my Nvidia cards (GTX 970 & GTX 1050 Ti) switch to PCIe 2.0. The Asrock AMD platform apparently doesn't like AMD cards!?
* only showed RX 570 results for an example, but the RX 590 was identical.