@schwanz9000
There has always been a problem with WalkType 2 (Boe, Chuck, etc) where the legs will hyperextend at the knee backwards and/or bend sideways. It only happens in-game and never when previewing the animation in Unity. Adjusting the length ratio between the upper leg and lower leg bones, as well as putting a slight bend at the knee helps reduce it but it doesn't fully stop it.
I just noticed, in Unity, that during the WalkType 2 animation, the feet are sinking below the zero plane with nearly every step. That combined with the Foot IK is forcing the leg to bend out of shape, which is why you see it only in-game. I tried raising the animation at the root so neither foot passes below the zero plane and it fixed it a lot but there is still occasional bending. I assume adjusting each individual foot and leg in the animation won't solve it either.
I think adjusting the Foot IK so it's constrained to only bend the legs in the natural direction will fix it. But that is pure speculation on my part.