How did you reach the belief that it had been moved to main?
Here's a way to be tricked into believing a post is on main when it's really on discussion. Take the URL and remove the "r/discussion" part; what remains looks like main post link, and works perfectly well. If your belief comes just from having seen such a link somewhere, it's probably spurious. One way to get such a link is if the post used to be on main and it was the correct link. But I think that they appear sometimes for reasons unknown to me.
On the todo list of features for the site is that such misplaced links should work, but change the URL to the correct one.
For the record:
If I go to someone's overview page, click the permalink for one of their comments, and then click the title at the top of the permalinked comment such that I'm back at the full article, it always shows up with this problem ("MAIN" in bold and related misleading issues).
Just tried it by going to army1987's overview page, clicking the permalink to here, and clicking the title up top. You can try following the link and clicking up to see the effect on this thread.
If I remember correctly, other kinds of shenanigans happen with user pages and comment lists, though in this case I think I understand why the permalink causes issues.
One way to get such a link is if the post used to be on main and it was the correct link. But I think that they appear sometimes for reasons unknown to me.
ISTR that Google searches (incl. the internal search at the top of the sidebar) sometimes return such links.
Because MAIN is bolded when I go to the link, as is typical of posts in Main. Posts in Discussion have DISCUSSION bolded at the top.
I didn't cut the "r/discussion" out of the link, but it's quite possible that someone else posted using such a link. The post may never have been moved out of Discussion, but it had scrolled off the page, so I jumped to the conclusion that the post had been moved.
In any case, I'm correcting my post.
It has a karma bubble that's white on the inside, like discussion posts, but the top of the page doesn't say discussion, and the word "MAIN" is bolded instead of discussion. Weird.
Edited to add: The post was probably always in Discussion, but if "r/discussion" is taken out of a url, the post looks like it's in Main but without proper notification. See the comments here for a bit more detail.
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LW Women: Submissions on Misogyny was moved from Discussion to Main, but doesn't appear in lists of Promoted, New, or (in the right sidebar) Recent posts. Comments to it aren't showing up in Recent Comments.
Edited to add: Comments are showing up in Recent Comments for Discussion.
I've brought this up twice-- in the comments and in the open thread-- and it doesn't seem to have made any difference.
I assume that this is an honest mistake or a site weirdness, but I'd really like the topic to get the attention it deserves. I'm also wondering whether other posts have gotten semi-lost the same way.
Edited to add: Is there a best place for bringing up site weirdnesses?