Yes, I've had the same problem and it looks like an unintended bug. I was attacked by a Spanish army where there was no bridge. The infantry could not cross, although the cavalry could if I aligned them just right. It looks bugged to me.

In my case I had two Spanish armies to fight, one on my side and a larger garrison on the other (but the one that attacked was on my side, so it was not a "sally.") I killed the one on my side easily, but the other could not figure out how to cross. So then I had to figure out how to cross it...after 15 minutes I finally managed to figure out how to get all my cav across, but my infantry was stuck. Luckily of me, the AI let me destroy it's cav and infantry piece meal (my cav were badly outnumbered.)