- Edited
Nizzo Many thanks to Nizzo. That hack worked. Dropping the user__cdata table did not work. The table was not regenerated and locks on-screen. Nizzo's approach stopped the errors being dumped and allow us to use the search criteria.
Adding detail here for devs. We force all tables/db to be InnoDB using utf8mb4/utf8mb4__unicode_520_ci format (international). Noticed that the newly generated table is MyISAM using utf8_general_ci. But the rows of user data are regenerated correctly in that table with the correct collations.