Yes, this is also my experience. Clicking “next” will take you to the photo that is next alphabetically, even when you have sorted the pictures on “modified”. Has this always been the case? And did I never notice before?
Afaik, yes. But I’m not a 100% sure because most of my photos have the date and the time in their file names like this… IMG_20200226_095812, IMG_20200226_095822.jpg, IMG_20200227_172405 etc…
(Btw, if you go to the “photos” interface (app) and you navigate to the same folder, click a photo, and then click “next”, you do get the next photo on the basis of “sort modified”.)