Continuing the discussion from Logstash 6.3.2 - File input plugin 4.1.5 stops writing to .sincedb after some time:
Just want to follow up on this - looks like the cause was due to a file handle leak: https://www.elastic.co/guide/en/logstash/current/logstash-6-4-3.html: Fixed leak of file handles that prevented temporary files from being cleaned up before pipeline restart #193
Which explains the behaviour we have been seeing.
After the upgrade to 6.4.3, the input has been running for days now with the same previous config and it is still running now so looks like the fix did it.