I have a serious issue with Euphoria mutation. It is cancelled after every combat. Even the DNA icon disappears from right side of health bar HUD.
Changing the mutation to another, and swapping back fixes the problem until next combat.

I get the problem even while not using any mods.

Are there any other workaround besides the manual mutation re-enabling?

Seems the bug happens with decoctions only. If you have a non-decoction potion active or drink a non-decoction potion, the buff reapplies.

Anyways, there's a mod already to "fix" it. Haven't tried it yet:

http://www.nexusmods.com/witcher3/mods/1587/?