I just recently got the Galaxy J2 Pure. When I was setting it up, I added .mp3 files to use for ringtones and for notifications. It originally allowed me to set these as the Notification sound for the Message app as well as custom notification sounds for specific contacts.
A few hours later, even though they still show as having the customized nofication sounds set, it uses one of the pre-installed notifications. Even when I try to change these, it show and will let me select the added sounds, but still plays the default sound. I can however still use the custom sounds for the alarms from the same folder.