Don’t set HipHeight to 1 because that’ll cause most NPCs to be -1 stud into the ground.
When you set the spawn-in Position just add 1 to the HipHeight you’ve calculated.
Try that and if doesn’t work then try Massless. If you make both changes you won’t know which affected it.
I only recommended it as a test to see if doing that cured your problem. If it did then you could try making the added amount smaller until you no longer had the issue. Even if just adding .1 to the HipHeight helped out then you’d know what was going on.
If an NPC spawns in 1 stud above the ground then they’d just drop to the ground and stand anyway.