I took a shot at tech support for your issue but unfortunately could not find a solid reason for what's causing it, let alone a fix. This is a real head scractcher! Hopefully someone more experienced can find a solution, but otherwise here is what I found:
I did see that the beehaw member that posted to your community must be using the correct URL to interact in your community, given that they went from their post at New Communities asking about the "404 error" to then "successfully" making a post in your community.
I then searched for any specific reasons that beehaw might have blocked interaction with your community but found nothing concrete. I don't believe beehaw blocking interaction is the cause, but they do state in their guidelines that they block things based on admin judgement and not on concrete rules, so it's at least possible this is the case.
I believe I saw you've already contacted the admins here at lemmy.world but have not received a response yet. Given that the lemmy.world instance is in its infancy and the people running it are still dealing with an influx of things to work on, my guess is they are swamped and might take a while to see your report.
My best thought for now: reply again to u/[email protected] on their New Communities post where you gave them the correct URL they needed to see your community instead of the 404 error they were previously getting. Confirm that they are able to see your reply to them on this New Communities post. Let them also know the issue with their post being invisible and unable to be interacted with on your community. Tell them you are awaiting response from lemmy.world admins and encourage them to please submit the issue for review to their own admins at beehaw.org. (If they don't confirm they can see your reply on New Communities, that might be invisible too. At that point, send them a message directly at their profile on https://lemmy.world/u/[email protected], identifying yourself as the community mod and conveying the same information/request for them to contact their beehaw admins for further support.
This all could just be code issues, server issues, etc., that admins are adjusting to with all the new traffic on the Fediverse, but I think it's worth reaching out to u/Witch to be sure they and their admins are also at least aware of the specific issue as well.