To better demonstrate the issue, I've created a test module called jsonapi_test_custom_entity_uuid which alters the entity_test uuid key to 'test_uuid' and created a test EntityCustomUuidKeyTest which extends EntityTestTest and just enables this new module which now fails on the ::testIndividual() test function.
Attached is both a patch with both the tests and the fix which passes tests and a patch with just the tests which fail.
Cheers
Tylermc → created an issue.
I've created a patch that adds the card-header div around the Fieldset legend and the card-body to the Fieldset wrapper div as they were in previous versions (4.12) while leaving the new features of the Fieldset that was added in later versions.