For example ethash has a decrease from 43mh/s down to 36mh/s after using WildRig. Doesn't occur if I don't run the application. (system reboot restores performance)
Any suggestions, comments on this behaviour?
This one is really odd. Did you try to reset(disable-enable device in Windows) Vega after wildrig session? When cryptonight was popular, people were doing so to get "fresh" hashrate, so maybe something similar happens here. Wildrig do nothing itself to modify anything related to gpu.
Might be, only have this GPU in this system, so device reset might not be usable, but could work for multi-gpu system.
I know driver reset are finicky and best avoided on RX VEGA though, bad recovery rate. Risk of instability causing crashes if you try to many times. Not the best auto-recovery in general.
I just expect this to be a feature of the code, I've noted and seen unexpected oddities from code structures when I was messing with that stuff years back. Simple changes could have drastic behaviour changes, even though it should make no difference.
I expect that maybe some resource is not released and kept reserved for the application upon closing. just a guess but worth to try and check.