Further digging shows this error to be caused by phpMyAdmin's default connection collation being set to "utfmb4_unicode_ci". Every new user that logs in to phpMyAdmin will have its connection collation set to "utfmb4_unicode_ci" by default. Unfortunately, that collation is incompatible with Debian's MariaDB server default collation of "utfmb4_general_ci". You cannot mix unicode and general collations in queries and views.
To correct this problem this package's default connection collation needs to be set to "utfmb4_general_ci", or the Debian's Maria DB server package's default server collation needs to be changed to the more appropriate "utfmb4_unicode_ci".
