When is dm_exec_procedure_stats updated?
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.
NOTE: An initial query of sys.dm_exec_procedure_stats might produce inaccurate results if there is a workload currently executing on the server. More accurate results may be determined by rerunning the query.

Resource
Is sp_recompile or WITH RECOMPILE being used? Those can cause the stored procedure to be removed. Also, it could be in regards to the name of the procedure. This answer talks about that.