Dude your card is throtling.check memory temps while mining.tteamredminer shows mem temps or use aida64 or hwinfo. While gpu core gets 65 maybe your mems getting above 100c thats why it is lowering power to cool down.
that is somewhat correct. I have 5700 (non XT sapphire pulse) running stable for years at 56MH, they pull 780-825mv, clocked at ~1300 core and ~1820 mem. the core is around +60 while the memory junction is about +100. Using old drivers 20.11.1(+no amd gui crap), mod with morepowertool and set volts/clocks with overdriventool. never use Afterburner for AMD - its shit, in fact I don't use it for Nvidia as well.