Not sure this means anything but, I had a special ringtone I like to use with Hangouts, for a day or two it worked fine, last few days it stopped working even though the app told me it was selected. Long story short, the ringtone HAD to be on my phone not off the card. I know someone mentioned that about messenger, so I think my issue with hangouts confirms that point. Keep in mind that I've put the tone I want to use in messenger in every notifications folder I could find
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Are you using Adoptable Storage like the OP?