Photos app importing photos out of order...

If you feed a Mac one picture at a time, then the import will be in order. If you give the Mac a hundred pictures, then they will be saved in a random order. This is the way it's always been, since Macs have had multiple cores. Since the Mac does parallel processing, different pictures are imported through different paths and arrive at different times. Their "Created Date" should be accurate, though.

So if you look at Recently Saved, then pictures may be in a funny order. But if you look at Library sorted by Created Date

rather than Added Date, then they should be in the order they were created.

In albums, Added Date isn't a choice for sorting.

This is a series from Library sorted by Capture Date-- the index numbers are in order of creation:

Here is a series from Recently Saved:

You can see that they are random

Is it possible that you have changed the sort order?

Richard.Taylor wrote: Is it possible that you have changed the sort order?

Perhaps in the past you looked in the Library or in Albums where they were sorted by Creation Date. The only thing new (about this) in Sequoia is that Library now has the option to sort by Date Added, as I had said. So let me ask again, Have you checked the sort order in Library?

Some common reasons are if the photos have lost their dates in social media like Facebook, WhatsApp etc. Or the time zone or location was somehow incorrectly set in the camera or some images had them and some not depending when the device happened to get a GPS signal after going outdoors.

Finding the culprit might need much sleuthing and usually it is easiest just to manually fix the dates so the images sort correctly.