Aside from a very brief banner that pops up when an attack is initiated by an enemy, there is no sign that shows your primarch is under attack unless you move off and try to put on more troops and get the message saying so. That or you find you have less troops/a dead prim.
It would be cool if maybe when a primarch is under attack, there is a visual indicator.
I love the idea of a visual from the primach menu where it shows which primach engaging in a battle. The only thing I worry about w extra visual is similar to flying against a defended base with that horrible glowing green everywhere where you can’t tell which flak tower is what. I don’t know if they can make it a clean visual for us phone user w limited screen space.