Camera sorting pictures out of order?

Most likely, that thumbnail leads to a list of photos in arrival order rather than trying to sort them into any other order. Just out of curiosity, does his Recents album exhibit the same strange order?

As it happens, I've been adding some older photos to my library recently. I just checked and can confirm that my Camera app lower-left thumbnail displays and my Recents album on my iPhone both display the recent additions near the "present" despite them being from past years.

So, yes, it can happen. If you want to view photos in chronological order by capture date, use the All Photos view. Use Recents if you want to view in addition order, which can be different. The view in the Camera app is great for the photos you just took with the Camera app but less good if you have been adding photos in other ways.

Just to confirm this - on my iPhone and iPad the Camera is showing the photos sorted by the date they have been added as well, just like in the Recents album.

And the date the photos have been added to the library is not stable - it will change, whenever the Photos Library needs to be rebuild after a major system update, that changes the format of the library. When Photos is rebuilding the library, the photos may be added to the rebuilt library in a different order, not necessarily the order the have been taken. When you add large batches of photos at once to a library, for example by air dropping them, they may be sent in a a random order, not necessarily in the same order as in they are album they are in.