Depending on the specific apps it is usually that it is not using GCM/FCM (something like Conversations) or that the battery manager is very aggressive and What

Depending on the specific apps it is usually that it is not using GCM/FCM (something like Conversations) or that the battery manager is very aggressive and Whatsapp is whitelisted by default (because so many use it). For Samsung I think it is the latter, since many apps have the same problem on Samsung.