Steps can also be seen in the screenshots attached.
Steps:
- Create two groups with the default settings
- Make these two groups, moderators of Your first forum
- Create a new forum under Your first category copying permission from Your first forum
Keep the rest settings default - View forum index, you only see the moderator list of Your first forum, not the second one
- Go to Forum moderators and select the newly created forum
- As you see, the two groups are assigned as moderators of the new forum
- Select any group of moderators and hit Edit permissions
- In the next screen just hit Apply all permissions
- View forum index, your new forum has at last the moderators visible
NOTE: The problem is NOT solved by just purging cache.