In this case I’m thinking three separate skull matches counted as three separate instances of “when I take damage”, which all successfully triggered a Giant Spider.
I don’t think that’s necessarily “bugged” so much as “not something that was thought about” in the implementation of the trait and skull converters, and is a bit eugh.
At least in this case the Orbweaver hero actually took damage rather than it being soaked up pre-emptively by the summons that needed the hero to take damage to be summoned in the first place actually no one taking any damage (Orbweaver probably had an undisplayed -1 skull explosion damage or similar), unlike:
I’d actually be very much in favour of all these damage sources being lumped together as one damage hit per cascade for battle resolution mechanics, meaning you’d only get 1 Giant Spider when converting multiple skull matches, although if that lead to a subsequent skull drop cascade, each of those could count for another source of damage.
This are a couple of other traits/mechanics that would be affected by this, they don’t come to mind currently, but I don’t think it would really have much of an impact, and I’ll come back and edit if I remember.