Gridnelwald and Dumbledore are exes (yes, I know Rowling said the feelingw were one-sided, but for the purposes of this prompt...)After Grindelwald's taken into custody, it's a big deal and the whole Wizarding council is invited to view the interrogation. Dumbledore manages to get himself invited or let in, planning to just observe. Of course, this turns into him and Grindelwald arguing about their relationship as the council and MACUSA officials listen/watch in confusion.
Bonus for something along the lines of
"Well, at least I'm not the one with commitment issues!"
"Oh, so you weren't the one who up and left without saying so much as 'I'm sorry'?"