you start out on the ground with marines surrounding you. this is the first mission where you have allies on the field. Chino hands you a majestic revolver and you get up and get to start the mission. (these arent what i think cause it but:) i backed up to grab the nanocrystals of the dead ceph that chino shot, then i grab the one that marines killed that i was facing. i then went to the left and jumped over a rock to go down to an alien body that didnt have any nanocrystals. i follow the marines down to the water where there is a bus, they go past the bus but then stop and get frozen there. sometimes one marine goes on without them a different direction but ends up going to where you're supposed to go and if you keep on going until you reach the ceph and start fighting, chino and his men will eventually get you you.

I think the cause of this bug is jumping over the rock and getting in front of the marines. to avoid this, just stay behind them and follow them.