It seems the 'type' => 'string' is not right
From the codex
type (string) – Custom field type. Possible values are ‘NUMERIC’, ‘BINARY’, ‘CHAR’, ‘DATE’, ‘DATETIME’, ‘DECIMAL’, ‘SIGNED’, ‘TIME’, ‘UNSIGNED’. Default value is ‘CHAR’.
try and remove the above line from the code and check or try using the below instead.
'type' => 'CHAR'