I have this same issue, on 18.12.3. Thanks for the HBCC memory segment trick, that worked for me, hopefully this bug gets squashed for good in the next update
edit: HBCC toggle seems to be a fix for only 5 minutes before the card goes back to 835/500.
edit #2: after a few days of generally fiddling around (unrelated to this issue) I replicated this bug after an extended gaming session (approx 2 hours). Still on the latest beta drivers, clockspeeds came down to 849 core / 700 HMB2 from overclocked 1600/900. GPU and HMB temps were always <60 degrees celcius so not likely a throttling issue. I'll do more 'research' into this and see if I can give you guys some more info.