OneDrive date modified without changing the file

While I agree that MS did not give enough care to protecting Modified Date in OneDrive (I too use it as a form of version checking and to find recently edited files, and it's way too easy to accidentally make a change and lose the real Modified date forever), I tried and was not able to reproduce the problem you described:

I created a new Word document on a local drive (not on OneDrive). I made changes and saved it, confirmed the Modified Date reflected the time of the changes. Then I opened it a few minutes later without saving any changes and only the Accessed time stamp changed in file Properties, not Modified.

Then, I copied the file to OneDrive. Opened it in Word again by double-clicking from Explorer in the OneDrive folder. I had not turned off Autosave, so it was even set to Autosave, which will provide the most aggressive updates to the Modified value. I moved the cursor around in the Word document and made various selections, but didn't change anything. Then, I closed the file and checked the results.

The file's Created time and date showed the time I created it on OneDrive (not the original creation date), which is how all MS OS's have treated Created since the DOS days -- always reflects the time it was added to a new volume/drive. The Accessed time showed the time when I last used the file. But the Modified date still showed the time stamp of the last actual changes to the file before I moved it to OneDrive.