Appears to be unsolved in version 1.2.2, with Curios 5.0.6.0, Forge 39.0.88

Abridged log from the pack I was making when I found this crash: HERE

Recreation in the minimal scenario outlined above HERE

In both cases, the stackoverflow is output repeatedly. For the sake of page load times, I've included one instance of the repeated log sequence in each case

Recreation: Summon way too many mobs (15 presses of an oak button onto a repeating command block does the trick - or just dump chicken in until they're all taking crush damage lmao) into a 5x5 cage, swing the haunted steel scythe into the horde. I could swear it happened with less once, but that seems to be a consistent amount.

I belive this has been fixed already, if you join the discord server or are already in it there's a 1.3 beta there. If you want you can check if it crashes there or not.