@cn It's weird since micro.blog (by design) mixes a given users' micro.blog-native posts with their self-hosted posts.
So, if you only Webmention the place a post originated from, only micro.blog-native posts would receive micro.blog Webmentions; replies to a self-hosted blog presumably wouldn't show up on micro.blog even though the (self-hosted) post you're replying to DID show up there.
Words are hard, hoping this expresses my confusion. Not sure what the right thing to do is.
My end goal is, regardless of where a post originated from, if it appears on micro.blog, I'd like my self-hosted reply to it to appear on micro.blog, too.