My photos have the correct time in EXIF data, but still show out of order. The first photo has a time of 2021-06-01 16:11:59; the second photo has a time of 2021-06-01 15:20:57. The third screenshot illustrates that I do have "Keep Sorted By Oldest First" set on this album.
MacBook Pro Retina
Check also the time zone of the images. If the time zone is different, this will cause them to be out of order.
Photos is only showing you a part of the time in the info. By default, the timezone is not shown. But the time 3:20 pm could mean anything plus minus 12 hours, depending on the time zone assigned by Photos on import. the two photos have been taken with two different cameras. If the timezone in the cameras has been set differently, the sorting may be wrong. Or of you imported the photos from the different cameras while in a different timezone the assigned timezone may be different.
I have changed the system preferences so, that Photos will show all date and times with a timezone suffix. That makes it easier to compare the times, see: How to display the Date and Time Zone in … - Apple Community
To correct the timezones, if they are wrong for one camera, you may want to sort your photos by the camera model with a smart album with the rule "Camera model is". This will allow you to select the photos from a specific camera and the batch change the timezone with "Adjust Date and time".
Photos out of order
