I have tested this and it appears to have been corrected IF you use Explorer windows. I cut and pasted from my local drive to SharePoint Sites and One Drive for Business and WITH THE ONLINE SITE open in explorer. That kept the modified date intact.
I cut and pasted and copied from One Drive for Business to a SharePoint site with both open with Explorer and it kept the information.
I copied and pasted form SharePoint to SharePoint, both open in Explorer and it kept the information.
It would not let me cut and paste from SharePoint to SharePoint site, so I have to copy then delete from the target site and leave it on the destination.
Obviously way to late to help Kit, but if someone else stumbles on this problem this might help them. Just use two explorer windows.