Message alert sounds

For some reason, when I receive a text message and I'm in front of my Mac, I receive the notification banner and alert sound from my Mac, but I only get an alert on my iPhone if locked (no sound).

If unlocked, I get the banner and sound as well on my iPhone, plus on my Mac, which is how it should be and has always worked.

I am now on High Sierra and iOS 11 (iPhone X) and am only noticing this.

Notification settings are correct on my iPhone.