I have one in my shop right now doing the same thing… Battery percentage does not change and the phone reboots constantly with the original battery and with a new one. Originally the phone came in with a bad power button so we changed the Power/Mute Lock/Volume button flex ribbon … which appears to have caused the new issues.
May try another ribbon just to eliminate that as the cause but Ive personally repaired hundreds of 5 series with bad power buttons and never had this issue afterwards.
WEIRD!!!
Will post results.