Yes – I should have asked those two questions above plus a couple more. Sorry I did not think of these yesterday…
@mr-papa asked about a temp switch to a WP default theme. I think the interest in that – and something I should have asked – is about the age and construction of your WP theme. In particular, is the main menu created using the Menu Builder in WP’s admin, appearance or customise menu? Once upon a time, this did not exist and menus were created using php functions with no UI to control them. Many of those themes still exist and work. This could be an important question.
The other question I should have asked is if you go to the forum admin > Integration > Page and Permalink – is the option to ‘Filter WP list pages’ turned on? And if it is not, can you do so and then see if that makes any difference?
Finally, if that does not change anything could you try something for me? IF the theme DOES use a standard menu, could you go to the WordPress ‘Pages’ and select the forum page for editing. As you are hiding the page name via the SP options – could you edit the page title and ONLY the page title. By default it would have been entitled ‘forum’. Change it in some way – for example – to ‘forums’ and save. (Yes – grammatically incorrect but useful for the test!). Does that fix the issue?