iPhone 7 is messing up the date stamps on some photos and videos

[Wekiva]

My wife and I both have iPhone 7. We record a lot of video with the two phones and with several other video cameras. We dump them all into a set of folders and then sort them by date to watch them chronologically. For some reason my wife's iPhone 7 is adjusting the date of about 1/4 of her videos and photos, which causes them to play out of order. I have to go back and manually change the date stamp on the files to get them to sort properly. I've updated her phone to the most recent update with no improvement. Any ideas?