She used a special spell to look like Juliette, and I assume it had a side effect of taking his powers away. Also because Juliette had to do the same thing to look like Adalind, to restore his powers. So my assumption is that she used a spell, and after they’re together she wouldn’t be casting it again.

It‘s officially said in the show that it was the spell. I really don‘t get the confusion. 🧐

Well, when Nick took Adalind's powers with his blood, his blood got into her and that's how she was able to use the spell to take out Nick's powers and it could only be done once. Elizabeth (Sean Renard's mom) told Monroe and Rosalee in order for Adalind to have been able to take Nick's powers, Adalind must have lost her powers at a point and then Monroe and Rosalee told her Adalind did and it was with the blood of Nick's.

As a result of sleeping with Nick, Adalind becomes pregnant, again. She gives birth to Nick’s son Kelly at the end of season 4/beginning of season 5. As a result of increased Wesen attempts to kill Nick, and as Juliette is now Eve (& no longer “with” Nick), Nick locates a new more secure place to live & takes Adalind & baby Kelly under his care. Nick has his Grimm powers back & does not lose them again. Slowly during season 5, they “fall in love” & begin to sleep together, literally & figuratively. I.E. Sleep & you-know-what. Adalind seems to have lost her Hexanbeast powers, again, or is not able to use them during the pregnancy & shortly after. It appears that Nick has genuine feelings for Adalind, and she for him. Odd, yes. Weird, yes, too. But that is it.

Adalind tested the potion that they created to suspend Julietts powers.