Apple TV 3 only plays stereo when subtitles are selected



I have a QNAP nas setup with a plex server installed, and I'm using iPhone/iPad to stream my content to the Apple TV 3.
I noticed that when I select subtitles the stream always goes to stereo, even though I used Handbrake to convert all of my content to 5.1 AAC, which is supported natively by the Apple TV 3. When I turn off the subtitles, it immediately switches back to Surround 5.1 and works flawlessly.

The question is, can this be resolved with the current setup, and if not, will the Apple TV 4K be able to stream te subtitles with the video? I'm getting a bit fed up with needing to convert every video I want on my server trough Handbrake because I want subtitles with working 5.1 Surround.

Thanks in advance