Why do LocalTemp table showing with lengthy underscore under Temporary Tables of tempdb in SSMS 2017 Express Edition?
The lengthy underscore followed by the number were added by the engine to your requested table name in order to make the table name globally unique in the tempdb database. You can query your local table in the session that created it using the #localtemptable name that you gave it. What you are seeing is perfectly normal behavior and there's nothing to be alarmed about.