Same problem for me too. Was driving me mad until I found this thread. I had converted an MTS video to MP4 and the "Media Created" property was filled in on the new MP4 with today's date, whereas it's empty on the MTS.
iCloud Photos uses this date over date modified or date created, so all my converted videos are showing as today!
And there's no way to clear the Media Created date, because you get the error as below: