If your team has motion tracking and you are looking at an alien, it won't show up on MT - that's normal, just a method to keep the blip out of your face. However, if an alien is in a marine's line of sight, it will disappear off of MT for the entire team. That's fine for the minimap, since they can see the alien itself instead, but it's inapproprpiate for the screen blip. What this means is that you can be watching an alien moving on MT, but if it turns a corner and one of your teammates sees it, the MT blip will immediately vanish from your screen. The criteria is exactly the same as when MT shuts off on an alien in your own personal view; the only problem is that this happens to the entire team as well.

I haven't tried it in Combat yet.