sys.dm_exec_procedure_stats missing history?
From the documentation on sys.dm_exec_procedure_stats:


  Returns aggregate performance statistics for cached stored procedures. The view returns one row for each cached stored procedure plan, and the lifetime of the row is as long as the stored procedure remains cached. When a stored procedure is removed from the cache, the corresponding row is eliminated from this view. 


Therefore, it is likely that some time on September 1st, the server had an event that caused the procedure cache to be flushed, or at the very least, caused the plans for the procedures you care about to be pushed out of cache. We've already ruled out the most obvious culprit - a failover or service restart - but it could have been anything, really:


a unit test that executed a whole slew of procedures, filling up the cache
an sp_configure change, which - for certain options - has a side effect of clearing the procedure cache 
a single query with a huge memory grant
a manual DBCC FREEPROCCACHE;
explicit recompiles or drop/re-create of procedures
...