It appears that when a r15 user has a package equipped, during spawn, parts are added to their character only to be immediately removed and replaced again. This bug appears to only effect server scripts, and not client scripts.
This causes issues with people trying to do things to the character when they spawn, such as welding a part to the character.
Example:
This would simply fall to the ground rather than welding to the players torso. The part the weld was attached to gets removed immediately and replaced with a second one, so it’s connected to nothing.