Galaxy S5 :: Why Notifications Cause Music To Play From Speakers With Headphones Plugged In

I have a really bad problem with my Galaxy S5. My S4 was fine until I updated to Lollipop, so I think that's the problem.

I'm playing music with headphones plugged in, Whatsapp notification sound plays through speakers.

That's fine.

BUT suddenly during notification, the music ALSO plays through the speakers for a few seconds! This is really frustrating because I receive messages throughout the day and don't want everyone around me to be disturbed with what I'm listening to (in library, on bus, etc.).

Nothing I try works.

Before Lollipop, Whatsapp notification would play on speakers and audio still through headphones. That was fine.