Hi again! Sorry for the delay (also again…).
Perhaps another caching plugin interfered with the correctness of the sitemap’s output — it might’ve fixed itself already. Do check before proceeding.
Now, I have no experience with LifterLMS, so I don’t know what I’m looking for — it has a whopping 172 000 lines of code, and since LMS software generally does not adhere to the WordPress APIs, I do not know where to begin.
I tried to follow their bbPress integration, but that doesn’t look like something I can integrate, and comes with a host of extraneous data that may cause memory exhaustion on sitemap generation (though, I mitigated that as best I could in TSF).
You may have more luck asking their developers; lines 29 and 30 of the gist I proposed are effective here.