To demonstrate the problem I made a clean installation of phpBB 3.0.RC7.

Steps can also be seen in the screenshots attached.

Steps:
[list: 31vtifta]

• 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.