Thanks for catching this. I suspect that it’s an issue with Sharkey, but I don’t know for sure. The reason this might be a problem is that some servers have sort of different rules for how strict they are about domain names matching between the various URLs that ActivityPub uses internally. In other words, for a custom username like @dan@danielmrose.com, internally Micro.blog still uses micro.blog URLs in some cases, and that might be tripping up Sharkey. Or it could be an issue with Sharkey following redirects between a custom domain name.
I’ll try to test this, but it might be a good thing to report to Sharkey too because they will be in a better position to troubleshoot.
Thanks @manton! I will definitely pass this along. I’m guessing you’re right because I had a similar issue with write.as. So it likely is something on Sharkey end.