2 possible workarounds:
1. Build a physics based pathfinding system and you can use this in place of the "mp_" movement functions
2. Make the AI a non-physics object and invisible - then create a physics object that is always lerping towards the AI. More info on this post here: https://www.reddit.com/r/gamemaker/comments/82a4q2/an_idea_for_smooth_looking_pa...