Tried to reproduce the error using https://github.com/pethers/hibernate-test-case-templates/tree/master/orm/hibernate-orm-5, but worked as expected even when I changed it to use 5.4.11 and Java 11. But will try to create a standalone example using model.external.worldbank.countries.impl in a separate git repo the next few days.

Only major difference I could detect from the test-case-templates
<exclude-unlisted-classes>false</exclude-unlisted-classes> and <property name="hibernate.archive.autodetection" value="class, hbm"/>, I specify all classes I use directly in the persistence.xml.