Elasticsearch 5.x indices in Elasticsearch 6

As per documents-
Elasticsearch 6.0 can read indices created in version 5.0

Index has a parent-child relationship with multiple types. I verified that data in the index created in v5 is read properly in v6. I also got the correct search results.

Next I want to confirm if documents can be added to the v5 index and searched by the same query as used in v5.

My findings - I could insert and search documents in indices but do not get the correct result with the query.

MY question - what is recommended? Are the indices of prior version read-only in the next version?

Adding documents to indices created with the previous major version is supported, maybe you found a bug. Can you share how you tested with us so that we can try to reproduce?