For the most part the skill seems to be working correctly but noticed 2 times in my recent game where it didn't.
1) Pioneer attacked by monster
2) Army attacked by mite army
Also, we were discussing the mechanic of this skill in regards to cities in another thread (towards the bottom of page 1 into 2 https://forums.elementalgame.com/441721/page/1/#replies). The skill says monsters will ignore units, not cities. Provided the skill does not extend to cities it seems to be working. However, if cities are supposed to benefit too, the skill isn't working properly. I had cities attacked by a lone Butcherman, Mite army, and an army of the crystal worms.