Custom oak:index for content fragment causing issue on query result

We are using a lot of content fragment on our aem 6.5 project. On backend we do search for content fragment based on path and datetime combination. Recently we have added a custom property oak index only for our content fragments. From the query performance tool we checked that query is picking proper index.

But the problem arrises when there is any update on content fragment data the query result is still the same. If we manually reindex the custom oak:index that we created, then it works again.

Also sometime the result is not as expected. But got fix if we manually reindex the specific oak:index.

What could be the issue? Are we missing something? I am sharing a screenshot here.

Note: The querybuilder console result and code result is always same for the same query.