Plain (not encrypted) push notifications are not sent for OMEMO encrypted messages without <body/> and for messages with <body/> its value is sent instead, ie. stating that your client is not OMEMO compatible.

I think it should be changed and notification should be sent, either without body but for priority high (if supported) or with body set to You've got an encrypted message. I would out for not sending body but mark it with priority high if it is OMEMO encrypted message.