This occurs only for the first comment on a wiki page, if the comment is posted via Quick Reply. Subsequent comments will be correctly styled. I guess XenForo still doesn't have support for its own "css" JSON output parameter that it has, at least not implemented during Quick Reply. This is unfortunate because all CSS for any usable BB-Code would have to be pre-loaded just in case the user might enter it in Quick Reply.
Anyway the post itself not being styled should now be fixed in the next release.