Why is my Photos app sorting incorrectly after macOS 15.4.1 and iOS 18.4.1 update?

Usually this is caused by a mismatch in time zone and/or GPS info in images shot with different cameras.

The usual cure is to in Photos fiddle with adjust time and timezone panel until the images sort as desired.

It might also help to enable time zone display in Photos via the Terminal with (re-launch Photos so it takes effect):

defaults write NSGlobalDomain AppleICUTimeFormatStrings -dict-add "2" "HH:mm:ss z"

...revert to the default time zone-less display with:

defaults delete NSGlobalDomain AppleICUTimeFormatStrings

See also:

Movie dates and Photos.app - Apple Community

As Matti says, this happens because "time" and "time zone" are recorded separately, and sometimes "time" even includes "time zone," so there's redundant information and different cameras do it differently. It's hard for us or our computers to keep up.

I agree that, rather than try to figure out exactly what's going on, the best plan is to "fiddle with" the menu "Image>Adjust Date & Time…" and try changing time zone or time, and being happy with whatever works.

Do keep in mind that, when you select a bunch of pictures and choose "Image>Adjust Date & Time…," you will see the time for the first picture only. But when you change the time, it changes all the others by the same mount. So, even though it might seem like making the first go from, say, 1:23:04 to 1:23:04 might make them all say that, but in fact, it will add 1 hour to all of them.