Samsung s5 Neo, version 6.0.1, default notification overrides custom sounds.

[AC Question]

Just got the Samsung s5 Neo and when I choose a custom ringtone for an application the cell will override this with the default notification sound. For example, when I choose a ringtone for twitter and go back to the main screen then go back into settings, the sound will have been automatically reset to the default notification. This is annoying and I want to change this. Please help.