In order to have Oversec correctly process what's shown in in the Messenger app, you need to enable the option "Scrape All Views" in the "Lab" tab of the Messenger-specific settings in Oversec.
This app has a limit of max. 2000 characters in the input field. For that reason, some encryption / encoding combinations may not work as they will produce (invisible) text that is much longer.
If you're having problems, please try one of the follwing: