I notice some time ago that, when posting a new topic in the forum, some tags that I was almost sure they were available before were not showing up when searching for them.
I’ve investigated further. This happens when you didn’t select a certain tag first amongst those ones that show up when you first click on the tags textfield and a dropdown with certain values is displayed.
In this case, it should prevent you from posting the topic without one of those required tags being present. It should also display an error prompt when you try it.