What happens if character with mythic haste is target of non-mythic slow spell? How about in case of lower level spell vs higher like mythic darkness vs day light? I have not been able to find any rules recarding the situation.
hmm, I see. No wonder I had no luck finding anything then... how would my fellow DMs house rule this?
I would not house rule it. It is working as intended. The mythic version is still the same level spell. It normally just has some extra benefits.