In the example I gave above, the transition is simple enter on a true bool, exit on a false one. As straightforward as you can get. I’ve done it a million times exactly the same way but it seems that for some reason I get this weird “incorrect” interpolation between the two animations only on those animations I exported using the biped rig.
I personally suspect it’s not a problem with the transition rules, but I agree it could be in the state machine. Maybe in some of the advanced settings of states?