Linked Parts & Explode Function

Hi all, I have this strange problem:
when exploding chords to new parts in score editor (but the same happens to Explode function in midi menu) these parts are linked together in a way that it’s impossible assigning them to different instruments. The new tracks are independent but the parts don’t follow. I tried Ctrl- U, K and I’ve seen many posts in the forum but I’m stuck.
Any clue? Thank you!