Extended Events session file truncated on server restart
Based on your question (and additional comments regarding your setting of max_rollover_files to 1:


  However it appears that a new event file is created when SQL Server is restarted and the history is lost. Is this expected?


Yes - that is expected.  When you restarted SQL Server all currently running extended events were terminated.  When you restarted SQL Server, a new extended event session started and your existing target file was replaced because you indicated you only wanted 1 file.  If you want to preserve extended event data stored in target files across server restarts, you will have to increase the max_rollover_files to something greater than 1.  

sys.fn_xe_file_target_read_file (path, mdpath, initial_file_name, initial_offset) has the capability of reading all extended event files in a the path referenced by mdpath if you specify null for the initial_file_name, so maintaining multple files should not be a problem.

Regarding the use of the Ring Buffer, take a look at why Jonathan Kehayias (world renowned expert in SQL Server Extended Events) Hates the ring buffer.