bbPress.org

It happens in the index page of the forum.

And EXACTLY it appears like:
Forum A
– Forum A
– Forum B
– Forum C
Forum B
– Forum A
– Forum B
– Forum C
Forum C
– Forum A
– Forum B
– Forum C

I think it is a sort of incompatibility between a plugin and the permissions plugin
bbPress Permissions -> https://github.com/ZanderAdam/Tehnik-bbPress-Permissions