CTT - test /authorize 'PAYLOAD INVALID SUB'

The CTT-test for ‘PAYLOAD INVALID SUB’ sends for sub this data: (“sub”: “invalid-value”).
In the examples on dev.ishareworks sub contains an EORI; however, the documentation states that the syntax is to-be-determined. So why should “invalid-value” be a failure?
My IdP implementation does not reject this value which is considered a FAIL in the CTT.
(see test CTT.7GHLMGKWY5, completed on 2020-02-21 15:26)

According to RFC010 sub should read “urn:TBD”; which is repeated in the title of this test-case. But if we ONLY allow TBD then the tests ‘VALID REQUEST’ and ‘ADDITIONAL PARAMS’ fail because they have sub= EU.EORI.NL000000001. Please explain!