My ringtone has stuck now for a couple of days.
I had the same issue (T-Mobile) with my 1st Note 7. The ringtones I downloaded were from Zedge. Seems to be working fine now with my replacement. I would suggest removing the SD card and then reinserting it (powering off the phone of course before doing so). Hope this helps.
I never had a ringtone on a SD card though. I've only used factory tones.
Will downloading audio pro manager and setting the ringtone through that app fix this issue?
Worth a try
It's not the best answer, but it works for now.
I started using Tasker to set my ringtone. I already have profiles set up for charging and discharging, so I added setting the default ringtone to both of these. Now my ringtone sets itself to the tone I want several times a day.