1. make sure Settings > Notifications > Facebook are checked
2. sent private message to test device from other Facebook account
3. lock device and unlock
4. Check notification
Result: No notifications are displayed.
5. Settings > Notifications > Facebook are Unchecked
6. Settings > Notifications > Facebook are checked again
7. sent private message again
8. lock device and unlock
9. Check notification
I can reproduce this issue on r88, steps as below
1. make sure Settings > Notifications > Facebook are checked
2. sent private message to test device from other Facebook account
3. lock device and unlock
4. Check notification
Result: No notifications are displayed.
5. Settings > Notifications > Facebook are Unchecked
6. Settings > Notifications > Facebook are checked again
7. sent private message again
8. lock device and unlock
9. Check notification
Result: No notifications are displayed.