Could you log a support Incident and let us know the INC number here on the forum so that we can track it?
I was able to identify one BUG-706396 (fixed in 8.6.4) involving problems when Enable search results for all properties is causing mapping mismatch for a field in different mismatch. IndexValidation failing while running report.
If Enable search results for all properties is removed and the field for "include search results" is selected then it has been reported as working.
Please be sure to provide all details in the incident: when the index last worked, any changes on the environment (updates etc), full pega logs, screenshots and details of the