Newly written SASI index files are not being included in snapshots. This is because the SASI index files are not added to the components (org.apache.cassandra.io.sstable.SSTable#components) list of newly written sstables.
Although I don't believe anything except snapshots ever tries to reference the SASI index files from this location, on startup Cassandra does add the SASI index files (if they are found on disk) of existing sstables in their components list. In that case sstables that existed on startup with SASI index files will have their SASI index files included in any snapshots.
This patch updates the components list of newly written sstable once the index is built.