Bottom line a good sound engineer with time and access to the raw audio could mix it to work well with stereo systems. But that ain't happening.
> But then other videos don't have the problem.
They likely would if they were intended for playback on a cinema system and just flattened to mono or stereo.
Christopher Noland doesn't direct/produce movies thinking about how they'll look on a phone on netflix. And the sound engineer he hired doesn't care either.
How much time and money was dedicated to the sound engineering to convert that 20 speaker maybe 100 virtual channel system to your 2 speaker system? Then they need to do it for 2.1, 5.1, Atmos... Whatever else is the new thing these days.
And if you have 5.1 did you consider carefully the layout? Are there multipath effects you haven't dampened? Did you consider the path to each speaker relative to listener?
> Does my sound setup know somehow?
Well yes, kind of. Your TV if it is at all modern will do some processing to the audio. If it's connected to a 5.1 system it might just pass through a 5.1, what does it do with a stereo audio? Or 2.1? What if it gets 7.1 it must decide what to do likely you can control this in the settings.
Let's say you have a 5.1 setup at home. And your TV sends it stereo? What does it do then how does it make 6 channels from 2? The subwoofer is easy, relatively non-direction a low pass filter sends all low end to it and high pass to the regular speakers.
My TV has a decent "clear speech" setting which picks out speech boosts them and then suppresses no speech audio around them. Usually it works perfectly.
Playing stereo audio on a stereo system will work fine.