iPhone push notifications cause screen blinks, without text message

The problem

Some APNS push notifications that I am sending from a server contain alert and sound, but they only cause the iPhone screen to blink momentarily, while no text is shown in the notifications section.

Settings & findings

• Python and PyAPNS on the server side


• iOS 8 devices, both iPhone 5c and 6


• Did not find any consistency - some messages pass and some don't pass


• The payload size is less than 256 bytes. Furthermore, the problems also happens on iOS 8 which has a 2k limit - so it's probably not a payload size issue

Any idea why my alert text does not appear in the notification bar?