This is a bit of information that might help someone.
I just spent an hour trying to figure out why my Room Divider Wall type would change from a single line to a dashed 4" interior wall when I connected it to another wall. Well, low and behold, if the material is set to Air Gap, this will happen. I changed it to Opening (no material) and all my Room Dividers went back to a single line.
For future reference, a proper Room Divider wall type in X17 should generally have:
0" wall thickness
Opening (No Material) assigned instead of Air Gap
No framing layers
Be designated as a Room Divider wall type
I hope this helps someone.