Thanks @morez.
The Follow party-function in GM-mode only makes the NPC port to the level/scene with the party. The NPC does not follow behind the party in the scene/level.
I am also in that thread you shared. But DeeZ, who is making that mod, hasn't been around there in a while, unfortunately, and the mod is not published.
Unfortynately I have not found any GM mod that adresses this.