assigned ringtones return to default at power off

I have an xperia E4 less than a year old, everytime I power off on start up the assigned ringtones for contacts have reverted to default setting and I have to set them again.
Two questions
firstly, anyone know why this happens or how I can avoid this happening again
and secondly, do I ever need to actually switch the phone off? will it damage it being on constantly?
many thanks in advance for all help offered

they are stored on sd card

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