Also ran into this issue. Annoying to have to go through every comp and delete these effects.

I've got the same problem... why has this version of AE been released with this tremendously annoying and time wasting problem.

almost a year later...

Workaround: after dynamic linking from premiere > after effects
1.) Select all footage
2.) Right click
3.) Highlight Effects > Remove All

(granted if this is all the effects applied from premiere)

Happens every time I use dynamic link (which is pretty much every project) and it's incredibly irritating. What possible reason could there be for adding two stereo mixer effects with volume levels at 35.57% to every layer? Why isn't this fixed yet?

@Andrei - Adding 20db gain to the final files at the end instead of deleting the stereo mixer tracks on each layer will severely compromise your audio quality and dynamic range. Adding 20db of gain to the files at the end will cost you 20db of dynamic range and won't bring back the lower amplitude audio that becomes inaudible/cut off with the initial loss of volume from the dual stereo mixers.

The same issue is happening to my projects.
For some years we've been doing the same workflow: importing edited ChromaKey timelines from Premiere to After Effects. Now, with the CC 2020 Update, the "Stereo Mixer" effetcs are changing my video final volume, so I have to apply a 20db gain to the final files.

Does someone have found the solution for that? Maybe changing the Premiere Timeline configs? Or something about the audio format, like Wav or Aiff? (My audio source is different from the video).

Isn't it an issue for you that all the tracks get downmixed :s ?