I am experiencing exactly the same problem. BP Group Moderators do have capabilities like edit, close, merge, split topics or replies in the Group Forum but they do not see the textbox to assign or edit tags. I tried to show the box by hacking the files
by removing the condition “&& ( current_user_can( ‘assign_topic_tags’ )”
but then the tags assigned in the box have not been saved, anyway.
There must be a bug in the capabilities mapping from BP group moderator to bbP forum moderator. The capabilities like ‘assign_topic_tags’, ‘edit_topic_tags’, etc. are missing there.
For BP group admins and ordinary members it works well, they can assign and edit topic tags in their group forum.
But where is the code of this mapping? I cannot find it.