Top speaker doesn't work on calls

[allseeingeye]

The top speaker on my HTC One M8 has stopped working on phone calls.

If I play music, the speaker works just fine. So it's not a hardware issue with the speaker.

The bottom speaker works fully. So someone I'm calling can hear me, but I can't hear them. If I put the call onto speaker that works too.

I've read a lot about related issues, but not this specific one. My phone and all apps are fully updated.