I have three ideas on why Hawkmoths powers don't make him detransform and want to know this groupes opinions on them.
1) His main power isn’t transforming the villains it’s creating/attracting the akumas and transforming them is just him cashing in the saved up power in the akuma while the “Evilization” thing is just a spell he learned in the book .
2)it’s revealed that ladybug will get a magic gem that gives her unlimited “lucky charms” and he may just have found his version of that
3) His power is still technically active until the akumas defeated and as such he just detransforms after his closing speech and we never get to see his miraculous countdown.

He is very overpowered even if he doesn't have a thing like that. Ladybug has the power of Creation and that means that she can make a thing once per transformation that helps her situation, Chat Noir can destroy one thing per transformation he has the power of destruction. Their Miraculous's are supposed to be the most powerful of them all but Hawkmoths seems infinite I mean if you can make one person do the crazy things that Hawkmoth can give people he is very powerful.
And I don't think that he has Infinite Akuma's he has never Akumatized more than one person, except in Sapotis but they were both touching the thing. you could also argue about Timebreaker but it was the same person and he hadn't Akumatized her yet so I think he does transform back but we just don't see it because we are watching the fight play out.
But if he did get something like that everyone would be in big trouble.
He does have to wait for negative feelings but that's not very restricting.
He might just be this overpowered because he's using the Miraculous Negatively, but he's still overpowered.

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What I think happens is that he can choose to transform back whenever, but is forced to once one of his akumas are beaten. Take when he akumatized Simon into Simon Says, he returned to his normal form to keep up appearances. Whereas when he akumatizes most of his akuma, he remains as Hawk Moth the entire time they're akumatized.

Now the whole "forced to turn back to normal" thing can't really be confirmed, due to us not really sticking around for what happens after Hawk Moth starts ranting about how he'll get the ladybug earrings and the black cat ring. But I think that is what makes the most sense.

I allways thought he did detransform after charging up the butterfly and just feed noroo and transformed back off screen