Think I have figured out what was going on. What is happening is that when an admin user navigates to a profile, the get an
'user_active_profile_id' set as the current profile. Because this is set the admin user is treated as the owner of the profile, they can update any item on that profile in the same way as the owner of the profile does.
The only exception is that if the admin user updates a Blog post it wont be shared to the timeline.
Its this thats getting in the way in the current situation. For forum posts the admin wants to be himself in this case, not an admin user accessing that profiles stuff. This case is different from editing a Blog or Audio or Other on the profile owners behalf. Its the admin user being a normal user writing a normal forum post.
I've added in some code in the forum/create page that sets the admin user back to themselves when creating a forum post. Its working. Im reasonably confident that the solution is the correct one.
The uncertainty area comes around if the admin user had multiple things open in multiple tabs on that profile. eg: admin user starts to write a forum post, opens a new tab, starts to edit a blog post from that profile, goes back to edit the forum post and saves. (who is the admin user being in this case.....)
I've checked it and cant make it fail even doing that, but if you can, let me know the steps. May not be possible, but also maybe.
Thanks Strumeila for the detailed description of how to reproduce it. That helped a lot.