Followup: The problem turned out to be a field in the mysql server
that was not size-limited to 255 characters and had several rows with
as many as 299 characters in them. Apparently when using an FDW and
materialized view in postgresql 10.4, field size limits aren't
checked, because a query on the matview in the 10.4 system finds those
records with more than 255 characters in that field.
So, not a character encoding issue at all.
--
Mike Nolan