I don't have a log handy, but Anor and I used them to pop a smob last night and one of our mobs throughout the entire combat neither engaged or assisted either of us. I assume it was likely my mob since I was not the group leader. However, I did still group all in an attempt to group the second mob and maybe trigger it to attack. This sadly had no effect.
Note that old mastermobs had this problem too, btw, so it's the assist coding that's off. The difference is that previous mastermobs could be ordered to assist. Also the first version we tried with this new system had this ability, but that doesn't really help much if the mobs stop following you and break, so... here we are.