This is about including a fix that was included in 3.11 and master only as part of a deprecation, in stables. The same bug exists in stables too.

The web service method "get_discussion_posts" will not properly exclude posts which should not be returned to the user. We didn't catch this before because there wasn't a test method covering this. When we final-deprecated the "get_forum_discussion_posts", we moved its test method over such that it was using the "get_discussion_posts" replacement method (which should work the same way), and the test failed.

You can find the commit which includes the fix here:
https://github.com/moodle/moodle/commit/4796ea753e0e703b4a93a76135877422e5703585

In addition to pulling the fix in, we'll want to include the test method included there too. Right now, this isn't a case that's covered in stables.