SQL Server Service has stopped, but process still appears in TaskMgr

A customer has the strangest situation. Windows Server 2016 with SQL Server 2016. There is only one SQL instance on the machine (the default). Service name: MSSQLSERVER. The service take a very long time to stop (about a minute), but once it is shown as Stopped, the SQLSERVER.EXE process still appears in the TaskMgr for up to another minute. If I try terminating the process from TaskMgr I get error: Access Denied. This causes problems when restarting SQL Server. This is what happens: - Stop SQL Server and wait for service to Stop (net stop mssqlserver) - Process remains in TaskMgr - Try to start SQL Server again (net start mssqlserver) - SQL Server fails to start because the Master database is currently in use Has anyone seen anything like this? Why would the process still be running after the Service has stopped?