I have experienced persistent issues with this on O365 Outlook on my Mac and now ToDo is acting the same way.
It's not consistent but happens very frequently and to many of my recurring tasks. An example is a monthly recurring task due on the 15th with a reminder on the 13th. I last marked the task as completed in Oct and it initially setup the next recurrence in Nov. Then at some point it will adjust the due date to Dec and the reminder to some random time on the 13th of January.
When this first started happening I could avoid the issue by *never* opening my tasks in the Mac Outlook client, and only viewing them on the O365 web site. That seemed to work. Then I started using ToDo and things were ok for a few months, but the same behavior has now affected my tasks as visible in ToDo or in the O365 web interface.
I'm using an O365 business account with the following up to date clients - Outlook for MacOS (have switched between New and Classic mode), Outlook for Android, ToDo for MacOS, ToDo for Android, O365 web apps, Outlook for iOS on ipad, ToDo for iOS
With this bug, it's an unusable solution that will drive me to another product with less integration with my staple O365 apps.