Yeah that's currently the intended behavior. The beetle fight is a 2v1 fight so it only checks the first 2 slots (from the left) if they have a spider assigned.
The Amalgam is a 5v5 fight so it checks for ALL slots, even if it finds only 1 single guy assigned on the last slot it'll still start the fight.
A bit of a... spaghetti code, not even sure how hard it would be to make it so the fight just searches for the first non-empty slot in your party, so you could still start a fight regardless if you assigned your guys to slot 1/2 or to slots 4/5