On AEM 6.5 adding an index does not let translation projects to start
While running some Translation projects, AEM logs reported "consider creating an index or changing the query".
We added an Index (oak:index) as below:

But adding this Index is causing the Translation jobs to fail ("failed to start translation").

Logs showing as:
java.lang.IllegalArgumentException: Could not find translation job with id 'test_translationjob_en_ja_da0d550a-dbf0-465e-9e26-7cf98cc1791c'
- While running a query using Query Performance > Explain Query tool, searching the the above job, results shows as the Job is Found and Index also being used.
- Removing the Index, is allowing the Translation Projects to start successfully.
Any reason why the jobs are failing with Index in place?