Adult child's return rejected for dependency status cross-check error

1. Parents joint return filed first, claiming adult child as dependent based on support.  Parents return was accepted.

2. Adult child return filed next, not indicating claimed as dependent on parents return.  Rejected due to dependency cross-check.

3.  Parents joint return amended to remove claiming adult child as dependent.  Parents amended joint return accepted.

4.  Adult child return did not to be fixed, since it never indicated dependent claim on parents return.  Return re-filed as is (no fix necessary) but still rejected anyway.  Tried several times with same result.

QUESTION

If the problem was fixed on the parents return, and the parents amended return was accepted, why is the adult child return still being rejected?  Note: no error code provided for the rejection.