Infantry units are getting stuck in an animation loop where they raise and lower their weapons indefinitely. This is pretty simple to reproduce. Place some OPFOR and BLUFOR units down on a map, very close and facing each other, and put this in their init line:
this setbehaviour "safe"; this setunitpos "UP";
Press play and you can immediately see this bug in action.
Here is a vanilla repro mission: https://www.dropbox.com/s/jdrddjbyagxqhit/setUnitPos_repro.Stratis.zip?dl=0