Hi Bill,
The messages (and there are several, from several different senders) are delivered to a Fastmail account as well as forwarded to Gmail, so I can tell for certain when a message hasn't made it to Gmail. I know about the Gmail->Fastmail->Gmail issue, so I send test messages from an external provider, e.g. Outlook.com.
I tried your test with example.org as well, and sure enough it made it through to Gmail. I guess that rules out guesses 1 & 2, and leaves some undocumented rule that silently discards some messages. Le sigh.
Peter
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