This is a frustration I've had for YEARS! I will discover a missed payment reminder because the Due Date changed on its own! Usually 3 months after the date I set, but sometimes exactly 12 months later. I will go in and try to fix all the incorrect due dates and later discover, always by accident, that some Due Dates changed again!
I use OneDrive to backup my Quicken data folder AND a separate folder where I put all my Ctrl+B Quicken backups. I use only one computer against these files. (Quicken would often complain upon launch that it couldn't open my data file, but that problem seems to be fixed now.) Another unique thing about my system is that all dates are in the following format: YYYY-MM-DD.
Today I discovered one particular reminder that kept getting pushed into 2028 when it should be 2027. I would fix the due date, close Quicken & Sync, and the same reminder again got pushed one year later. In desperation, I turned off syncing (0 accounts are synced now). THAT FIXED IT! I almost never use the Quicken mobile app on my iPad, but I would like the freedom to do so. Is this a known issue? Is there a fix? Is there another reason why Due Dates are changing on their own?
Been using Quicken since the first DOS versions in the 1980's. It was bullet proof then, but that trust has been gone for many years now.