Its intended to put it simply. If a character has the highest % in the Pi chart for Celica (especially if its more than Celica herself) they can end up having more control over Celica than you do. Its not permanent but it can become a challenge to regain control, especially if you gave that character some of the fox shrine spirit charms to have more power over Celica.
You can usually still escape and regain control, but for the characters you meet later into the game the tougher it tends to be. The rough thing with the dark world characters is that (pretty sure at least) you can't get kicked out of Celica's body. This tends to be the easier way to regain control, since there are guaranteed ways to regain control from that point.
So for Jade and other dark world characters, you just gotta get the timing right a few times in a row with struggling, and it can take a bit of time.
Edit to add: I've tried this for a few of the main world characters and since Violet is one of my fav interactions I got stuck with her in control for like 5 mins of combats too since she doesn't kick Celica out of her body. This was when i realized your situation can happen lol.