Fess search index loses docs

(from github.com/reversecipher)

I have a problem where Fess search index seems to lose its documents after a week. When Fess and crawler are started, the index (fess.YYYMMDD) is created and populated. However, after a week the documents disappear. The crawler is still running but the docs are not populated to the search index.

(from github.com/marevol)
See Remove Documents Before.