postgreSQL parsing json string field

The field'_app' in the process_issue table is a json string type

SELECT cast(_app as jsonb)->>'updated' app_updatetime,  _id AS id, _id AS title, _full_text AS CONTENT, _app AS app, _files AS files,info AS info FROM
process_issue WHERE _files != '[ ]'
and cast(_app as jsonb)->>'updated' > '2020-09-06 07:01:42'