Why can't i set my phone on silent and still be able to ajust instagram's volume?

[AC Question]

Recently uptaded my HTC One M9. Since, I can’t set my phone to Silent mode and get audio from Instagram at the same time.
I used to be able to have my phone on Silent mode and adjust the sound on instagram with the volume up and down buttons, when watching a video.
Now if I want to hear the audio, I need to put my phone either on Vibrate or on Normal mode. Can I fix this or is it a new “feature”?

Thank you!