When the "push-back" effect is applied under "area" action, a target is not pushed back while the others are.
(Edit: I forgot the vanilla push-back effect is already under an "area" action, so this summary is useless. But I think you can find the real cause.)
Steps to reproduce:
1) Download the following mod. What it does: it overrides the attacks of defender and destroyer by area push-back effect.
To show the effect is really applied to the targets, the destroyers also creates explosion entity on each target.
Here is the script:
2) Start a new game.
3) Summon some defenders and/or destroyers and make a trip to attract some biters.
4) Observe when the robots "attack" the biters.
Same result with or without "affects_target = true".
Could you also fix the algorithm of push-back so it doesn't calculate collision of the pushed entity against itself? Mentioned here