What memory timings show up in Polaris Bios Editor v1.62? All of my Sapphire Nitro+ RX 580 8GB cards have dual timing straps Samsung and Hynix. The Nitro+ cards also have a dual Bios, so you should be able to make a backup of the original Bios by moving the Bios toggle switch.

https://github.com/jaschaknack/PolarisBiosEditor