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?
Rx 6800 should have hashrate similar to Rx 6800 XT,in fact in many websites they report the same hashrate.I have lately build a rig with Rx 6800 Xt cards and the reported hashrate is the same from 5.5c up to 5.6b which is 59.8 Mhsh with default settings without any fine tuning and without playing with the values in AMD Radeon software or MSI Afterburner.I am running them in Windows 10 clean ISO downloaded from Microsoft.
My cards are Radeon Powercolor Red Devil triple fan and they run really cool and quiet.