Copy fails to replace "Date Modified" on existing files

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I did a manual copy of 4 files as one function from W10 to WD My Cloud using File Explorer to replace 4 existing files.
After the copy had completed, one file had its ‘Date modified’ date updated to match the source however the other three that were also copied still had the old mismatching ‘Date Modified’ values.
This causes my backup program (SyncFree) to believe that the files are not the same and so it copies the same files to my cloud every time it runs even though they were ‘copied’ in the previous run. As its backing up 30 or 40 files that are 1or 2 GB files this takes many hours.
Also the latest version of Setfiledate does nothing.
Any ideas as to why a complete copy is not being performed?