Carlos said
try changing the name of the forum in the menú… 

I would be interested if that fixes the issue. I had a similar problem on a site and changing it to Forums made it display properly.

Now to figure out why.

very small number of folks reporting similar… WP has had a long standing bug with the wp list pae function and we have had a hack in there to work around the bug…  hoping, though missed the ticket, that maybe they fixed the bug and our hack is no longer needed…

SOLVED

1. Deleted the page (forum) in Menu
2. Delete the page from pages – empty bin.
3. Create a new page – Forums
4. Add it to the menu – and adjust the name to FORUM

Done

In the picture you can see that I am inside the forum, and that it is also visible in the top navigation bar.

I am actually working on this at the moment and yes – the quick and simple answer is just to change the name – actually doing so on the menu definition is good enough.

But it will be fixed up in the next update and apologies for the problem.