I've never found Windows (all versions) to be consistent about this. I can't find any specification that tells us that the folder date should change if a file changes in that folder.
In fact it would be a bad idea from a reliability standpoint to change that entry with each file change. And add to the overhead and reduce the speed of the file system.
Have you found a MSDN or Microsoft document that says it should do this?
This began happening out the blue. In Details mode, folders do not update the date when files contained within are added or modified. It used to update fine. Any suggestions?