Constrain to Plane - issues with AI


I have a bit of an issue concerning constrain to plane.
My AI was working well and behaving as expected on flat brushes.

I then added terrain that has some slight angle variations but nothing where the character cannot walk.
I then started experiencing some strange issues.

1 Constrain to plane no longer applies as the AI starts deviating from its straight ahead vector and moved to the right at 0.1 degree difference. As a result in a few meters he is already off course. Not ideal for a platformer.
2 Due to the above after a while he is actually walking on areas that do not contain navmesh data
3 because i do checks by looking at its rotation they now fail because the character is no longer with in the acceptable deviation in rotation

So i guess my question is why does constrain to plane to apply to AI?

It does as long as your walking surface is 100% flat.

Not sure if this is a bug but if you have custom selected and not a specific axis then it does not work even if you specify that it is constrained to X