What clued me in that something was wonky is that the breadcrumbs for that forum failed to output any text, so when viewing a thread in that forum it displays the breadcrumb as:
Code
"SiteName > Forums > Category>  > 'Thread Name'"
                               ^ missing forum name

However, I just noticed that when at the forum level, it is even weirder:

Code
"SiteName > Forums > Category>  > 'Forum Name'"

                               ^ forum name should be here, right?


I suspect that this is related to why it doesn't show in Control Panel under Content > Forums.

Other forums do not have this issue.

Jesus was an architect previous to his career as a prophet
All of a sudden, I found myself in love with the world
So there was only one thing that I could do
Was ding a ding dang my dang a long ling long