My deleted messages on viber keep coming back

Hi,

I am using viber on my IPhone 4s and windows 8.1. Normally on viber, when you delete conversations on your primary device it will be automatically deleted on your secondary devices. But sometimes when I delete my messages or conversations from my primary device, they keep coming back as new messages a few hours later or the next time I connect to the viber. Is there any possibility that my viber account is hacked? How do I know which devices are connected to my Viber account?