I think I have solution for you. Whenever monster has reach ability, that means you can use it to hit enemy 1st position even from 2nd position of your team. Hitting opponent at 1st from your 1st is normal.
Meaning you should have swapped places of your Manticore and Beetle.
Cheers!
Yessss exactlyyyy......the positioning of my manticore was not right, nice one for catching that!