After a succesfull copy in explorer the timestamps are equal but somehow Robocopy copies the same file the next time it runs and claims source file is newer. Seems to be a problem with the smb version on the nas.
Adicione a opção /FFT
ao Robocopy:
ROBOCOPY /MIR /FFT <source> <dest>
/FFT : Assume FAT File Times (2-second date/time granularity).
/FFT uses fat file timing instead of NTFS. This means the granularity is a > bit less precise. For across-network share operations this seems to be much more reliable - just don't rely on the file timings to be completely precise to the second.