Sounds similar to the mining intensity issue with RX 6000 cards on windows 10. The Devs might have set the -mi value too high for your card. Try lowering the -mi value until you just start to lose Mh/s, for my RX 6800 it's about -mi 4.
What average hashrate do you end up with for your RX 6800? Have you tried the April driver + 5.6b?
I have:
Gigabyte OC 16 6800XT - 63.8MH/s using 1314/2140 @ 618mv 134w
PowerColor Fighter 6800 - 61.4MH/s using 1314/2140 @ 618mv 117w
PowerColor Red Dragon 6800 - 62.727MH/s 1314/2140 @ 618mv 104w
using 5.6b April Driver - driver only install