A new version with better full text indices has been deployed. Even though it has been tested several times with production data, there seems to be a bug in the indexing process.
I am working on it and will update the post.
UPDATE: The bug seems to be somewhere in the clustering technology (called Orleans). I am in contact with the developers from Microsoft. It only happens with more than one server. Therefore I have scaled down the cluster to 1 member yesterday to index most contents and then scaled up again. New contents might still not be visible via full text search.
UPDATE: Issue has been fixed and it was actually a bug in the clustering system but there was a workaround that has been implemented.