Not working when user is member of just one group

Hello,

I have following scenario:

(1) my config: drupal 6.13 / og 6.x-2.0 / og_forum 6.x-2.0-rc4 / og_content_type_admin 6.x-1.1
(2) my site has e.g. 2 predefined groups (created by admin). Let's call them "Group A" and "Group B".
(3) users shouldn't be able to post "group posts" (the default), only "forum topics" in that groups. Both these content types are "Standard group post (typically only author may edit)".
(4) therefore I have set the options for those two groups in "OG Content Types" to Require for Forum topics and NONE to Group posts.
(5) the default settings in "OG Content Types" are left also only to require Forum topics and here also the Allow ot Require checkboxes for Group posts are UNchecked.
(6) users have permissions to "create group_post content" and to "create forum topics"
(7) in "Organic group configuration" Audience required is set to required.

In this scenario if a user is not a member of any group he should be able to start posting of Group posts, but as audience is required and he isn't a member anywhere he will not be able to submit his content.

Here comes the strange thing. If he becomes a member of one group (Group A OR Group B) and starts posting Group post content then this one group appears in the Audience section, he is able to post the content and the content appears on the group home page!?

If he becomes a member of both groups (Group A AND Group B) then everything works as it has to, the audience section becomes empty and upon saving the message comes, that audience is required.