Search results mismatch after postgresql update

Today we had to update the PostgreSQL version on our database servers. Before update the version was 9.3.4 and the new one is 9.3.6. Despite the small difference in the database version and the absolutely identical configs, the search results changed drastically. The number of results decreased many times for some search queries. The rows added today (after update) were found each time, while some older ones cannot be found by a given phrase.

We use tsvector to perform full text search.

Any ideas what might be causing this?

Example query:

SELECT * FROM content, to_tsquery('bulgarian_utf8', '{keyword}') q
WHERE (tsv_title @@ q OR tsv @@ q) ) AS c