There are a few places in the jsonapi tests where there is a reference to issue #2940339. It states that the testRelated test should be re-enabled when that issue is fixed. This didn't happen.
I tried just removing the skips, but it seems there is still some work to be done to make sure the code in testRelated can handle NULL. Currently the tests fails as follows:
The 'data' member was not as expected.
Failed asserting that null is identical to Array &0 ().
/home/localcopy/drupal/core/modules/jsonapi/tests/src/Functional/ResourceTestBase.php:789
/home/localcopy/drupal/core/modules/jsonapi/tests/src/Functional/ResourceTestBase.php:715
/home/localcopy/drupal/core/modules/jsonapi/tests/src/Functional/ResourceTestBase.php:1377
/home/localcopy/drupal/core/modules/jsonapi/tests/src/Functional/ResourceTestBase.php:1296
/home/localcopy/drupal/core/modules/jsonapi/tests/src/Functional/CommentTest.php:396
I added a patch with the tests enabled inline to make debugging easier.