I have applications with LZ1 attachments that are FT-indexed, and searchable by their attachments. I would imagine that there is a limitation for FT indexing based on compression type. The FT index may be limited to indexing Huffman type or LZ1 type but not both simultaneously. 

You may want to make a database copy of the application, and run compact -C -ZU to convert the Huffman compressed documents to LZ1, and then full-text index the application. If the FT index then is able to find documents before and after the date you found to be problematic, then do the same for the application with documents of a mixed compression type.