I have had this issue as well since getting my 6S on Sept 25th. In the first 3 weeks it happened 3-4 times. I went to the store reported it we did a system restore and it was ok for a while. At the 6 week mark I went in to the store again because it had happened 3 more times. In 6 weeks it happened many times so they gave me a new phone (WHICH I REGRETTED - the iCLOUD backup was not working correctly and my messages did not come over as they had when I went from my 5-6S) I worked with representatives for hours doing different backups to the phone from iTunes and to no avail it did not work. I backed up to my 5 back up to get "most" my messages back except still missing 6 weeks of stuff.
I noticed this morning the battery icon percent issue happened AGAIN. I have contacted APPLE in a chat to just report it, they want me to try different things but I refuse because I have done all this enough I just want to report it.
She said there is a 9.2 update so maybe that will resolve it. She mentioned turning the feature (%) off and on again.
I see many people state about changing the date/time for games - that might correlate because I did that last night playing a game and this morning the % is stuck. I cannot say for sure because I do not know about the about 6-7 times.