I can confirm this also affects Jack’s Pumpkin Bomb; in the current Grimforest event, I took these screenshots:
On the LEFT pic, you can see I have applied Jack’s Pumpkin bomb to the center target. On the RIGHT pic, I have used Jarvur and Agnes to kill the center target, which results in a Reflect message for the other two red reflecters, even though I have NO red on my team.
I know this is a known issue, I just wanted to provide some clear screenshots and documentation. I believe it is simply treating the damage as coming from a red source (the center target) since that is where the Pumpkin Bomb is attached and generates damage when it is destroyed; rather than being considered a Purple damage source from the original hero (Jack).
It doesn’t cause any damage to the two enemies on the sides; but it also doesn’t cause any damage to MY team; it is just negating the Pumpkin Bomb damage from the others and presumably reflecting it back to the already-dead enemy in the middle.