This could well be expected behavior, but I've had more than a few defense runs where Brutes will be attacking walls and my mortar will knock them through the (unbroken) wall, allowing them to directly come after my defenses. Has happened to me on offense as well as I have witnessed my brutes being knocked inside the walls surrounding a defensive structure, then having to fight their way out after destroying the structure and moving on toward a new target.
I keep having the same problem. I spend a fortune on nice walls and than my mortar pushes them right over walls. Very anoying
Sometimes it also pushes troops away from what they are attacking (like a resource storage) and they start attacking a benign building. So it has benefits for the defender as well.
