So I'm having this behavior and only want to confirm that is the way it should be work.
I open wazapp, receive a message and the usual tone, then while the app is open if I receive another message it's a small bip with a short vibe, but if i left the phone and the lock screen is active if I receive a new message there's no loud notification, so if you have your phone in the pocket or out of you sight you won't notice you have a new message.

This is how it should work? Or is a bug?

Please can someone tell me.

Regards.