Steps to re-produce:

1. Create a 'Text Field' custom field type
2. Create 2 issues (e.g. TST-1 and TST-2) with the following value on custom field.

3. Go to the Issue Navigator and perform a search by putting the 'A-NA' in the 'Text Field' custom field

As a result, it returns both of the issues (TST-1 and TST-2). However, when I try to search by putting 'B-NA' in the 'Text Field' custom field, it only returns the TST-2.

The problem seems to have disappeared when I changed the Indexing language (Administration -> Global Settings -> General Configuration) from 'english' to other language and performed a full re-index (Administration -> System -> Indexing).