I had something similar happen yesterday.

I opened a thread about pokemon, browsed it for a bit, did some stuff in other tabs, and clicked back to the pokemon tab maybe an hour later to browse some more.

The post had changed to one where a user was asking for relaxing game recommendations and it was loading in new comments that seemed to be from that post, but I could still see the comments that had already loaded from the pokemon post when I scrolled down.

When I refreshed it changed back to the pokemon post and only showed comments from that.

There's an API section1 in the Lemmy documentation that describes some basic information and links to other resources.
The LemmyHttp class2 in the linked lemmy-js-client documentation lists out the endpoints, parameters, and return values.