Again, I am not sure at all, but yes, they runaway, the problem as I said is that they get too close, imagine you have an Ennead and a Sabus, both of them would attack the target from the same distance, even when their speed are totally different, like if they could put safe of the explosion in a equally manner.
The AI doesn't make difference based on the ship max speed, or neither on the enemy mass, it becomes commons that some type of ships gets caught by the shockwave and others simply not.
Shivan Comm Nodes explode quite fast after it's HP reachs 0%, plus the shockwave has a considerable range for it's mass which usually damages your allies, in mission 15 there are 3 Comm Nodes, so if the first don't kill them, the second will do, or the third, this usually don't happen with other Classes, like Cruisers.
If someone has doubts about this, you can just destroy all the enemies on screen, fighter and bombers mostly, and then start using your squadron to order to destroy the Shivan Comm Nodes one by one to test how efficient they are.