Another cause is the Smart Pixels battery saving feature that is present on some custom Android ROMs; ie Resurrection Remix. Disabling Smart Pixels will solve the problem.
In Resurrection Remix the setting is in Settings > Resurrection Tools > Interface > Smart Pixels > Enable Smart Pixels. Note:This setting is Disabled by default.
FYI, since no one replied I went ahead and tried it myself - OTA upgrade from cr-8 DID NOT WORK for me, it softbricked my phone so be prepared to use adb to sideload, factory reset and restore from backup!