It seems the Diagram tries to display a piece beyond the range of defined pieces at some point; such piece codes are interpreted as fusion pieces (a remnant of Paco Shako support). No idea why and when it does this; I will look into it.
[Edit] OK, got it. It happened when you click on an 'hole' square, as if you want to move with it. The piece that you last selected gets displayed in the one-line piece table, but the hole has an encoding outside the range of normal pieces, and so it was resulting in a request for a fusion piece.
It seems the Diagram tries to display a piece beyond the range of defined pieces at some point; such piece codes are interpreted as fusion pieces (a remnant of Paco Shako support). No idea why and when it does this; I will look into it.
[Edit] OK, got it. It happened when you click on an 'hole' square, as if you want to move with it. The piece that you last selected gets displayed in the one-line piece table, but the hole has an encoding outside the range of normal pieces, and so it was resulting in a request for a fusion piece.