One of my cards needs 910mV voltage (gpu-z) to run 1408/1100 stable, other can handle 900mV. If yours is stable - you are the silicon lottery winner ))
It still running in the same session and I don't see any speed fluctuations, it seems stable. As a matter of fact the pool is reflecting my hashrate very well too. Try lowering the HBM voltage to the same voltage as Vega 56 version. I think this is what did the trick.
BTW, I was able to compile dev-branch of xmr-stak-amd on windows ... they have one-letter bug in config that makes an error ... but I found it ))