MSSQL Master db singleuser in sys.databases but multiuser in sp_helpdb
First, 
Run the 

ALTER DATABASE master
SET MULTI_USER;


and confirm you receive the error Option ‘MULTI_USER’ cannot be set in database ‘master’.

Check your errorlog for any information that could explain why and when. 
Then 


Backup the master database
Restore as a different database, example 'master_temp'. 
Switch master_temp to MULTI_USER. 
Backup master_temp. 
Restore master with the backup of master_temp.