Question Regarding Audio on Sony Receiver

I just got a Sony STR-DN1080 receiver. It's hooked up to a Fire TV Stick 4K and Samsung U7000 TV. I'm also using a Sony UBP X-700 Blu-ray player. My question may have more to do with the Fire Stick since it APPEARS sound is working properly on the Blu-ray.

When I'm using Netflix, the sound seems to be okay, as far as the proper Dolby or whatever being used. But if I use another app on the Fire TV Stick, one that pretty much just sticks to basic two-speaker audio - like Tubi, Kanopy or Hoopla - the sound on the receiver is always "surround" and using the 5.1 setup. Shouldn't this default to just basic 2.0 sound since that's the way these apps are sourcing their material and many of these movies are older, pre-Dolby stereo mixes anyway?

I haven't messed with any of the settings on the Sony receiver, other than doing the initial calibration when I first received it. I'm thinking this may have to do with the Fire TV Stick's audio settings, which is set to the default "Best Available." Again, I'm pretty sure the Blu-ray's audio settings are registering correctly, though I haven't tried playing an older one- or two-channel DVD to test this.

Any suggestions?