When test cases are invalid (with bad XML) and got checked in to the DevTest repository causes issues in DevTest Port. DevTest Portal will not show any test cases (valid/invalid).
when a wrong test is present in the folder the XML parser used for getting information from test file throws an exception and whole REST calls just return an empty collection and causes the issue.
If you are seeing the similar issue, please open a support case and refer DE372137. Support might have a patch for this issue in DevTest 10.3.