So you get higher hashrate with 6 instead of 8 threads that leads me to belive that you är running your rams in slot: A1,A2 or maybe B1,B2.

Most of times its channel A1 & B1 it A2&B2 so that you get dual channel and not single channel.

For the records my 14nm ryzen 1600s so 3800/4000 hashes @ 3.3ghz

So Yea check your motherboard manual for correct slots for your ram sticks.