When opening a DataDraw document that has been set to automatically update on opening, you will see a Data Import Status notification. Assuming that there's one DataDraw object in the file, if the data file has NOT changed since the last time the document was opened and saved, the notification will say "1 graphic was already up to date." If the data file HAS changed, the notification will say "1 graphic was successfully updated."
When opening an up to date file on the other platform, however, the graphic will always "update." In other words, if you save a file on a Macintosh and close it, then open that file with ClarisImpact Windows (without changing the data file) you will see a Data Import Status notification of "1 graphic was successfully updated".
While it may be inconvenient to wait for the graphic to update when it should already be up to date, it should be noted that the resulting file is correct.