The text keeps disappearing because of a conflict between your static backup and the live iCloud Sync; while the backup restores the original text, iCloud immediately detects that the latest version of the note on the server has that text deleted and updates your phone to match the moment you connect to the internet.
To recover the text, you must restore the backup again but immediately turn on Airplane Mode as soon as the phone reboots to block this sync process. While offline, copy the recovered text and paste it into a new note, which iCloud will treat as a fresh entry rather than an old version to be overwritten.
Once you have saved the text in a new note, you can safely turn off Airplane Mode, or alternatively, if you have an iPad or Mac that has not connected to the internet recently, you can retrieve the text from there before it syncs the deletion.