I'm not sure this is going to help, but here goes anyway. I have an album in Photos on my Mac with nearly 500 photos which I've sorted alphabetically by title (manually - because, unlike in iPhoto, that's the only way you can do it in Photos). When I sync this album to my iPhone using iTunes, the photos remain in title order (NOT date order). So far, so good.
The 500 photo album was a bit unwieldy so I split it into four smaller albums on the Mac (titles A-G, H-L ...etc), each sorted alphabetically by title, and then synced these to the phone. On the phone, the four smaller albums lose the alphabetic sort and appear to be in date order. The 500 photo album remains in title order on the phone.
So the answer is that photos are NOT always in date order on the phone.
However, the conditions under which the sync decides to use title order or date order on the iPhone are a mystery to me.
Jeff