I had the same issue. I found a workaround, In the Settings WIFI Choose Other... enter the Name: SSID Name of your wifi, Choose Security then touch Other Network Button then enter password of your wifi. I did an envistigation myself to confirm if such problem is solved. On my laptop I did a ping to check if iPad is still reachable during sleep mode I encountered Request time out when iPad turns to sleep mode (which literally means my iPad is in offline mode) however when I send message using my iPhone (facebook, iMessage) to the iPad I get the notifications right after the message is sent. Hope it make sense. Thanks 😀