Well the first wave alway does I believe (based on some testing). That's why I talk about a switch to standoff mode.

: I wonder if their normal "charge forward"
: behavior is the same as their "go to the upper level of the cargo
: bay" behavior that they get when you trigger the second room
: checkpoint, but for some reason gets triggered differently?

That first one is what I was calling 'attack mode', but I don't see any link with the later behaviour of going to the second room (is it a cargo bay actually?). Not sure what you were getting at there - or if you were implying any link with your opening comment.