Jsyk, I ran into the same problem you had.
tl;dr
For semi-complex Actors (namely, characters), I suggest you DO NOT play animations from anywhere other than the AnimBP. I’d only use Play Animation node for very simple actors; things that use only a few animations. For these characters you don’t need to create an AnimBP. Just play all their anims in their main BP.
I had a basic state machine that handled all the animation states. Then I came up with the great idea of playing a single one shot animation in the Character BP. Then I noticed my character never reverted back to his state machine animations. I think is the same issue you’re seeing. I don’t think it’s documented anywhere, but I’m pretty sure whenever you play an animation from anywhere outside of the Anim BP, the Character’s anim instance gets set to Null or something, so it’ll never revert back to it.
Lastly, Anim Montages are extremely powerful, but I’m pretty sure you don’t need one to get what you want done.
Can you post some screens of your setup that’s not working?