What order did you click on the files in the operating system. Let’s try a test:
Here’s me clicking on 1.png, then 2.png, then 3.png, in that order, and uploading them:
Looks in order to me..
Let me try again with different images, and I’ll click on them in reverse order, 3.jpg → 2.jpg → 1.jpg
Not entirely sure what happened there, that looks like 2, 1, 3 to me.