I am running the official code example found at the bottom of the page here:
https://www.sfml-dev.org/tutorials/2.5/audio-streams.phpI have attached two audio files. "sound.ogg" is the original sample, consisting of a simple 440 Hz sine wave which only plays in the left ear. "smfl_result.ogg" is SFML's resulting playback once that sound sample it used with the above sample code. Note that there is very significant crossover between the two stereo channels, and some distortion is also present.
Given this bug is consistent between both my own code and the example code on the official website, it's clear that this is most likely a bug with SFML itself. I am on Ubuntu 20.04 with Pulseaudio.