Queries on string type fields not work correct

We work with elastic search. .Net, SQL server using "Nest client". A month ago i made a server upgrade from version 0.90.0 to 1.7.3, Nest client was upgraded from 1.0 to 1.7.1. I made some necessary changes in our infrastructure the "Nest" upgrade enforce me to, made backups & copy my indexes.
We made a lot of tests (on the test env.) & every thing looks fine ... while we made the upgrade on production we found we have problem (only on production!!!) with "Strings" searches. while we search on "string type" field we get million of hits, on our test env. its work correct ...
I made all the tests via external clients (sense etc.) directly on the server/cluster so my infrastructure/Nest client is not a part of this issue, i check my index mapping and its the same mapping on my 2 QA env. & production...

Is anyone had this kind of problem ?
Please advice.