my phone cannot play stereo over 3.5mm headphone jack
I found a quick fix for this issue, I think that every Xiaomi phone that's on MIUI12 can do the same step, the steps are:
on your phone go to...

Settings -&gt;   Additional Settings -&gt;   Accessibility-&gt; [scroll down] Mono Audio(turn off this)

on my phone, Mono Audio was (ON) by default, and that is the reason that your phone will get ridiculously loud (both on your headphone and speakers) and not accurate for sound direction while gaming or watching movies, notice if you Disable Mono Audio your over-all Sounds volume will decrease a by half, and AFAIK this is because your Audio volume will combine while on Mono.
hope that helps. thanks