Hard to tell what is up with the info provided. Can you reproduce this? Have these users use a default style and see if it still happens:
Create a new style with no parent:
- Styles & Templates > Style Manager > Add New Style
- Parent Style: No Parent Style
- Title: Default vBulletin
- Allow User Selection: Yes
- Save
Set this style as pickable by users and have these users use this style and see if the problem still happens.