You could be right I guess....I mean the thought has crossed my mind too - however just because it's been going on for months now, doesn't mean it's not an iTunes proble though - does it? I still feel it's some sort glitch with the photo cache file that was applied via either an iOS update, Window/Mac update, or iTunes update itself (which has my vote) & who's to say that it didn't happen back then in Dec/2017? That's when things went south for me too.
Prior to syncing - if you delete the photo cache file, reboot your computer then attempt to sync photos to your device - you'll find the photos, or at least some photos, taken/added since beginning of 2018 WILL sync to your device. It's the slightly older photos - dated 2017 or before - that sparatically if at all - get uploaded during the sync. This doesn't seem to fit the theory that Apple is trying to corral users into using that iCloud photo bit. I think it would be just opposite with old photos being applied with the new ones being excluded, but who know? Just my opinion.
No, I think something went very wrong with an update that affects the photo cache file, as well as utilizing dates of these photos as some sort of marker that limits accessibility to them. You'll notice that if you follow what I said about deleting the photo cache file, etc. - iTunes TRIES to sync the photos - i.e. you'll see the actual number of photos it should be adding to your device, (copying 100 photos of 2,000 for example) but then iTunes just doesn't actually add them, or doesn't finish the sync.
I just hope we either get a fix to this problem soon, or at least some sort of explanation. For this problem to be going on for so long is ridiculous.