Well they cured him of the zombification at the start of season three, and he just experienced a few after-effects: increased stamina and strength, ability to go for long periods without breathing, with his heart barely beating, etc.
From what I remember of early season three, this was initially an unconscious response to stimuli that he couldn’t really control. But as time went on, it’s possible he grew used to the power and learned to control it, the same way he eventually seemed able to filter out his super-hearing unless it was needed in a specific situation.
That being said…that’s ALL headcanon. They really didn’t do much with that idea once they got into the latter half of season three. I know his super hearing showed up a couple of times after that, but I don’t think the stoneform power ever showed up past season three.
One theory that’s been floated before in fandom is that the whole reason he developed, for lack of a better word, actual superpowers each time some kind of wacky magic was used on him was that one of the adaptations of the Grimm line is not only a certain level of immunity to some poisons/forms of dark magic (i.e. Nick being able to resist things like the Coins of Zakynthos)…but also the ability to incorporate anything that would make them stronger.
So when Nick gets turned into a zombie–cold-skinned, super strong and fast, never-tiring monster that doesn’t need to breathe–and then is cured of the ill effects, his body somehow manages to hold onto the useful aspects of the condition. Same story earlier on with his super-hearing. He lost the use of his eyes, so his ears just upped their game almost immediately. And then after he gets his sight back, they just kinda…keep going at that higher level without adjusting back down.
All of that to say that it’s possible when Adalind worked the spell to strip him of his Grimm powers at the end of season three, he was also stripped of most, if not all, of those adaptations as well.
And then when he gets his powers back, he either has lost all of the powers (not likely, as the hearing remained), takes some time to learn how to access them again, or loses only the purely mystical ones but keeps the super-hearing because it was a physiological change in response to the loss of his eyes rather than the side effects of a semi-mystical infection of sorts.
Make sense? Kinda? Maybe?
