Yes - lists are known to Hyperkitty but no, no Archived-At header in received posts.
But, Bingo!
I hadn't looked at the log file immediately after trying to post a message so I missed this. I don't know the solution yet but ... what is this telling me? Something doesn't have the permissions it needs somewhere...
Jan 02 18:01:15 2019 (616) 127.0.0.1 - - "GET /3.0/lists/mailman3_test.udfcd-lists.org/member/udfcd.alert@gmail.com HTTP/1.1" 200 484 Jan 02 18:01:18 2019 (609) ACCEPT: <20190103010115.530.63813@udmailman> Jan 02 18:01:18 2019 (615) HyperKitty failure on https://udfcd-lists.org/mailman3/hyperkitty/api/mailman/urls: <html><title>Auth required</title><bod$ <h1>Authorization Required</h1></body></html> (401) ... (that line repeates 4 times) Jan 02 18:01:20 2019 (606) Exception in the HyperKitty archiver: <html><title>Auth required</title><body> <h1>Authorization Required</h1></body></html> Jan 02 18:01:20 2019 (606) Traceback (most recent call last): File "/usr/lib/python3/dist-packages/mailman_hyperkitty/__init__.py", line 154, in _archive_message url = self._send_message(mlist, msg) File "/usr/lib/python3/dist-packages/mailman_hyperkitty/__init__.py", line 210, in _send_message raise ValueError(result.text) ValueError: <html><title>Auth required</title><body> <h1>Authorization Required</h1></body></html>