Since updating to Marshmallow, my phone won't save pics edited in the gallery.

Like WTF. It was working on Lollipop but now since Marshmallow none of my picture edits are saving. After I save an edited pic my phone will flash the circular loop like it's saving that I get taken back to the original picture without the edited picture saving.

I did another factory reset thinking something didn't install correctly, but it's still doing it.

Check the permissions for the photos app. I had a different problem; it was caused by a permission not granted after update.