Most of the time my infantry dismounting works the way you described Sergei, it is only every now and then that I have this problem. Whenever it happens though, I have to basically keep that 1 infantry squad and vehicle back from assaulting across long distances.
I normally force myself to work around it whenever it would happen. I just kept getting annoyed by it this time though, I was playing the first TF Thunder mission experimenting with arty call ins and timing mounted infantry assaults correctly and that 1 squad just refused to do it correctly no matter how many time I would reload and change the orders...