This is a guess based on some observation from the last few weeks.

It seems to me that new user messages need to be validated by an admin, and this causes the new messages to go un-noticed.

I started to look at that because I noticed some messages did not appear in my feed reader, and the reason seems to be that when the new message is validated, it still uses the original date of the post, but since more recent posts have been posted since and read by my reader, it's considered to be an old message and so it's not added in the list.

I haven't confirmed this (I just added some code in the reader to keep the xml files so that I can verify what's happening the next time I see the issue), and another possible reason would be that the new post is added in the atom feed but the date is before the <updated> tag from the previous time I checked the feed.

Another possible issue is that if there are enough posts on the site, the newly validated user post would just no appear in the atom feed.

A possible solution would be to set the <updated> field of the new post in the atom feed to the date of the post validation which should make it appear in my feed.

I know it sounds like a "me" problem and it partly is, but even when I come to the site and go to the home page or the /feed page I don't look past the last post I've already read and so I still miss those posts. Which means that changing the updated date (which is the same as editing a post in my understanding of how the site works) might no be enough if that doesn't bump the post on the main page /feed page. So maybe it would be needed to change the actual post date.

I only noticed this post https://handmade.network/forums/feedback/t/8526-information_regarding_project_submission#26781 because I went into the feedback forum, I wouldn't have seen it otherwise.

I don't have any posts missing from my rss reader (FreshRSS). The one you linked is there.

Your analysis is correct. The post/update date is based on the post itself, and not on the approval time.

Which reader are you using? I use FreshRSS, and when an older post is added it still appears as unread to me. I'm guessing your reader uses one read date for the entire feed, rather than tracking individual items?

I'll try to come up with a solution that doesn't lead to too much confusion. For the website we could maybe sort by max(post_date, approval_date) or something, but for the RSS we would probably just have to lie about the post date.

I'm using my own reader, which I will fix when I've got the time.

At the moment it checks if entries <updated> date are more recent than the last feed <updated> date (the one from the last time it retrieved the feed). So entries "in the past" are discarded.

I don't know how the site works but if the approval could maybe add a "post edit" (like if the user edited the post) with the approval date, it would changed the <updated> date in the atom feed. Or set the <published> date of the feed entry to the approval date, that should work too.