What is the reason?
Unknown yet. Will introduce feature to shut down miner in such situations so simple script can get it back working.
I have the same problem with 2xVega 64 and 4xVega 56 in one rig with windows 10 and amd blockchain driver beta. After a couple of hours sometimes one or two GPU's show 0 sols/s. stopping lolminer and restarting it resolves the issue. However I think the driver also resets on those cards as the memory frequency is back to stock settings, so I have to adjust it again for the cards that showed 0 sols/s. didn't have time to run further tests yet.