This was a bug that left it cached on the device. Apple and Google have put themselves in the middle of most notifications, causing the contents to pass through their servers, which means that they are subject to all the standard warrantless wiretapping directly from governments, as well as third-party attacks on the infrastructure in place to support that monitoring.
If you don't want end-to-end messages made available to others, set your notifications to only show that you have a message, not what it contains or who its from.
If you don't want end-to-end messages made available to others, set your notifications to only show that you have a message, not what it contains or who its from.