That cutscene triggers when you reach an exit from the map and happens regardless of your choices - not only regardless of you using the tadpole, but even meeting Shadowheart and the weapon.
IIRC Larian has stated that they want to make sure you have the funky box no matter what you do.
Ah, BG3's version of Solasta's
1. Take the Crown 2. Take the Crown 3. Take the Crown 4. Take the Crown
Noted. I sure love this kind of storytelling...
/s And here I thought it was actually related to my ingame choices. Well, you learn smth new every patch.
