Be careful with the 'read_and_close' option. It doesn't update the session file's last modification time unlike the default PHP behaviour when you don't close the session (or when you use session_write_close explicitly).
Old session files (for me, older than 24 minutes) will be occasionally cleared by the garbage collector (for me every 09 and 39 minute of every hour).
So a session can disappear even if the page regularly sends requests to the server that only reads and closes the session.