There’s a subtle difference in what I’m suggesting that’s different to what you’re doing, although I can’t confirm because you haven’t posted any code but it sounds like you are using the point when the first montage blends out to fire the second montage using tick to track the change of Montage_IsPlaying to do this, this is no doubt were the problem is, because the blending out of the first montage has already begun before you start the blend in of the second which results in the T-Pose.
Typically you would just need to call the second montage to start playing and it handle the blending transition. which is why I suggested a timer because you can get (montage length - blend in time of second montage) and then the timer start the playing of the second montage and it handle the transition itself as long as blend out/in are closely matched.
This is how I normally handle playing two montages one after another and I have never had it T-Pose before, well not due to code anyway 