This is not a bug, as far as I know, it's just that the logic of groups in forums is necessarily complex to handle forums that change modes or people that change groups.
The rule for a forum in groups mode is:
students can only reply to discussions started by people in their own group. Read that again.
Any discussion that someone starts is marked as belonging to their group.
One exception is for an editing teacher who can look at different groups. In this case, the group id of any discussion they start is set to whatever group they are looking at at that time. Thus, a teacher can post "into" different groups to seed discussions.
A teacher can also post a single announcement-type discussion to ALL groups (what we call group zero), but in this case students can not reply to it. Why? Because then students from different groups will be mixed together in the same discussion, and this causes all sorts of visibility problems later on if the forum or groups should change.
(If you need to mix people together then just don't use groups for that forum!)