Why can I not delete pictures or videos from internal memory?

[AC Question]

I have a Moto G3. I recently updated to Marshmallow and ever since I have been having problems with photos saving to both the internal memory and my SD card instead of just the card (which is the option I chose in the Camera app settings).

When I go into my internal memory to delete the photos, it gives me the option but then when I press the trashcan icon says "unable to delete some documents". If I delete the photos from the SD card, however, they also disappear from the internal memory.

I've tried deleting the photos by connecting the phone to my laptop but the folder shows up empty (whereas I can see all my photos on the SD card and move/delete etc.).

I also tried a factory reset but the same thing still happens. I currently have to keep only the minimum amount of photos/videos on my phone so that there's enough memory making having an SD card useless.

I currently have my SD card set as portable storage. I'm reluctant to try formatting it as internal storage as that's not what I want to do (transferring between devices is a big plus for me) and I have no confidence that it will solve the problem.

Would really appreciate some advice as I've got no more ideas left.