Also, this appears to be very kbin.social specific at the moment at least. I went around and checked kbin.cafe, kbin.melroy.org, fedia.io; disabled federation on each but all of them had their own domains shown for local posts. It seems only kbin.social is currently stripping domains from new posts made to kbin.social, causing the inability for any kbin.social users to view those posts if they block any domain (to clarify, this affects kbin.social posts FOR kbin.social users. You won't be able to see this issue from a remote instance, as the posts do show as the domain kbin.social over on say, fedia.io. It's only on kbin.social that posts to kbin.social are losing the domain)
Basically, this was fixed in the source code a month ago, but kbin.social itself (the only instance experiencing this issue) doesn't have the fix loaded in yet.