You have an extra scenario, first of all! 
Setting a primary group in Discourse can automatically set a user’s title. This is achieved through the “Default title” option in the group settings. However, there are a couple of things to note:
- The group must also have the “Automatically set as primary group” option enabled.
- If the user has already manually selected a title in their user preferences, it won’t be overridden by the group’s default title.
@awesomerobot shared this screenshot, showing where options are on the membership pages:
No idea why that is happening… make sure to check the settings of the groups involved, perhaps they don’t all have the same “Automatically set as primary group” setting configured.