https://tildes.net/ is a forum site similar to Hacker News and Reddit. It's not a massively popular site so its results don't always appear in organic searches, but there have been cases where I would've expected it to appear in the "Forums" lens.
So I figured I'd try to test it, and here's a regular search where Tildes appears in organic search (but I've raised Tildes in my settings):
https://kagi.com/search?q=Three+Cheers+for+Tildes+%28Android+version%29+alpha&r=se&sh=bYQaVjZS1750vDhQR4qebQ
However, if you turn on the "Forums" filter, it does not appear. Even when scrolling to the bottom of the results, it is not there:
I cannot share the search when searching with lenses, but the query is "Three Cheers for Tildes (Android version) alpha" and the region is Sweden.
Is there any documentation or details about how the "Forum" lens works, what sites are included, and how sites are added to it?