iTunes 12.9.5 not syncing photos from Photos App to iPad

Okay, I tried restoring the iPad just to see if that would solve it, and it didn't. But I did try something else afterward that DID solve it:

With the iPad still connected to iTunes, I then opened the Photos app. Once open, I selected my iPad in the left-hand column and then, to the right of that, next to the Device's name, I checked the box that says "Open Photos for this device". I immediately switched back to iTunes to check the syncing options under Photos for the iPad, and "Photos" was displayed on the drop-down tab next to "Copy photos from:...." I then connected my iPhone to iTunes, and this time the Photos option was already populated for me under "Copy photos from:...." without having to check the box for the iPhone in the Photos app. Problem solved! But thank you so much, Joseph, for helping me narrow down the options!

Thank you, Joseph, for your reply! I will answer your questions about this perplexing problem in order:

• I've never used iCloud photos on either my iPhone X or my iPad Pro; it's always been turned off. I've always synced photos directly into the iPhoto/Photos app on my Mac. Weirdly, I can still pull photos from my iPad/iPhone with Photos, and it imports them like normal.

• When I toggle the "Sync Photos" off with either device, it asks me if I want to remove the photos from that device, but if I select the "Remove Photos" button, it does nothing. I have tried shutting down Photos, iTunes and my Mac several times, but it didn't change anything.

• Installing photos from iTunes worked just fine on both my iPhone and iPad, right up until I upgraded to Mojave from High Sierra. Since that time, I've been able to sync movies/music to both devices with iTunes without issue, so I never thought to check on photo syncing until a couple days ago.

As an aside- I'm thinking of erasing and restoring both devices from the latest iTunes backups (starting with my iPad) to see if that helps. Please let know if you need more info. Thanks!