Not receiving Belle of the Ball Trophy

I had the same issue when I chose to fight the Dutchess (defend the empress) rather than preemptively confront and arrest her.  On my second playthough, I chose to confront instead, and the trophy clicked for me.

I don't think it is simply about your approval rating.  There are many different outcomes to Wicked Eyes and Wicked Hearts.

I think the margin for retreiving this trophy is very narrow.

I also had 100% Cort Approval several times. But the time spend from the courtyard until you walk through the last door leading into the ballroom can grab a point or two. I think it's very hard to get this trophy to be honest.

I've just attempted it, I though that you had to get the Empress, the Duke and Briele to work together with at least 90% approval.  I had 99% but it still didn't trigger the trophy.

The only way to get 100% is to save before the counter drops and reload if it does, that resets the countdown.  Maybe you have to not detain Florian?