Hello,
I had same issue with iPhone 5c, after replacing the battery I would experience the phone rebooting randomly and never changing percentage until a restart.
I have since fixed this issue and I believe the cause was the replacement battery I had was 1560mAh while my original battery was 1510mAh. Since replacing with with a 1510mAh battery the phone no longer reboots and the percentages increases/decreases correctly.
Hope this helps.