I have two armies standing right next to each other, but when the enemy attacks one of them they do not reinforce each other. I started a new campaign just now and it happens all the time. It also happened in a city. I raised a new army and ended the turn. A hostile army attacked the general who was in the city but the city did not reinforce my army so it ended up being one general unit vs the enemy army. I was able to retreat and they just sieged the city which i defended in my own turn but really what is going on.
As far as I can see into this it only happens when I get attacked and not when I attack myself, because when i siege a city the reinforcements which didn't work just one turn earlier do magically work again. Same with the city. I run back to the city with my general and defend it against the sieging army and the reinforcements work.
Here is a screenshot for everyone to see exactly what I mean:
Does anyone else have that problem too? Is it because I play on legendary? Or is this normal because I never realized that before and I played enough of this game already so I would've realized.