Tell me who knows. It used to happen sometimes, but on a zombie mission at MAGMA it became a pattern. When zombie soldiers or mega zombies approach, the reaction attack does not work in some specific cases (even with full TUs and 100+ reactions).
In the case of the megazombie, only one pattern was visible - the reaction was triggered only if it had less, approximately 1/4 TU. He could go straight into the face of my soldiers and dogs and only the shots were fired on time.
I tried to deal with the zombie soldiers from the corner after the doorway, but he could easily run another cage forward, turn around and make several attacks until the TU ended and only then there was a retaliatory attack. I put both the dog and the man in a row, but both stood motionless. However, when I put the dog in front of the door on the cage deep into the building, it calmly began to bark and stopped this zombie.
And at the same time the question is why very often barking is not activated when enemies run through the cage from the dog's nose. Marking priority for reaction attacks has no effect at all in this case and, it seems, in many others. If they come close, there will be a bite in any case.