I can't find "uid" as a part of any X500 syntax.
I looked primarily in https://tools.ietf.org/html/rfc2256
RFC 5280 says "Where it is non-empty, the subject field MUST contain an X.500 distinguished name (DN)."
I wonder if the UID breaks something. Just guessing wildly here, haven't checked any code yet. But it is interesting to me. Do you have any pointers to where UID is part of a DN?