Images have an older (correct) timestamp within the Photos app but exported files show the current date and time
At least with Photos v3.0, it does export the photos with the current date and time (which is undesirable) but the photos still have the original (correct) timestamp inside the EXIF data. That sounds like it may differ from your experience, so go ahead and check with Photos v3.0

If when you do export your photos again they still retain the correct EXIF timestamp but show the wrong Created timestamp on the file, then try using "Better Finder Attributes" app to first copy EXIF date to creation date, then to copy the creation date to the modification date.
(I have no affiliation with the "Better Finder Attributes" app, I'm simply a user of it).