I have had very similar issues.  I have never used the app, yet it was connected to one of my clients, that has never used the app.  Bills have been changed to different amounts and have "melio change" written in the description.  I just now manually disconnected the app.  Hopefully this will solve it.  Personally I find this an unacceptable glitch.  My clients pay me to handle their bills and QuickBooks accounts.  If it were to continue, I would have no choice but to start advising them to use other software.  Security must be an absolute priority.