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/Witch@beehaw.org 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/Witch@beehaw.org, 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.
IMPORTANT UPDATE, PROBLEM IDENTIFIED: At some point yesterday, beehaw.org decided to completely defederate from lemmy.world and sh.itjust.works in addition to the other instances they already cut off from. I believe the announcement came in later than when it happened which is why we couldn't figure out what was going on.
It turns out the four admins for beehaw are also the sole four mods for every single beehaw community. By that design, they are completely outmatched at moderating every beehaw community with the entire Fediverse able to participate and decided to cut off from federation in order to minimize the large workload.
It's now impossible for users from beehaw to interact with lemmy.world users. Even if you comment on a post in a beehaw community, the beehaw users there will never see it. This unfortunately means also that the beehaw user that joined your community will never be able to create visible posts or comments in your community from that account.
Here are some posts that really get into the details and explain it completely:
I want to add that I hope this doesn't discourage you from building your community. It will still grow and I'm sure your subscribers appreciate you creating a community for them to participate in. And any users like the beehaw subscriber you had will hear about the announcement and create an account on a separate instance if they want to rejoin. Thank you for helping build up the Fediverse, don't let this get you down!
Back again, coincidentally I just came across a similar post where someone commented with a potential solution. It's not quite the same situation but the comment implies that all comments, etc., will be synced over from the beehaw instance if someone from beehaw.org subscribes to your community at lemmy.world.
Looks like you have 16 subscribers at the time of this comment. Take a look at your subscriber list and see if any are from beehaw.org accounts. If there aren't any, request someone from beehaw.org to subscribe and see if it fixes the issue. If there already are people subscribed from beehaw.org, go with my original idea to just have those users contact beehaw.org admin in case it's an issue that can only be fixed on the beehaw side.