Same audio whenever I remove vocals from a song

Hello there, and thank you for taking the time to read my complaint.

My issue is this: there is this one song that I work a lot with that I import into my projects as an MP3 file.
To work with it, I have to remove the vocals, which is pretty easy.
The first time I ever used Audacity I was using this same exact song in a project, removed the vocals, then tried to do something else but ended up messing up the audio and made sound really low quality and grainy! So I deleted that project - it was an experimental run after all, since it was my first time using Audacity.

A month or two later, I decided to do a new project, with the same song. I removed the vocals, and that low quality and strange sounding audio from the very first project came back. All I had done was remove the vocals, and nothing else, and this random effect just messes it up! So I deleted the original project, and deleted the MP3, then re-downloaded it again. Import the song into Audacity, remove the vocals, and the same problem still happens!

This is infuriating. I have not an idea why Audacity instantly uses the incorrect, low quality audio every single time even though I deleted all traces of it. Anything anybody can do to help?
Thank you very much, and take care!

How well vocal-removal works depends on how the original was recorded : in some cases it’s not “easy” to remove the vocal, see … http://manual.audacityteam.org/man/tutorial_vocal_removal_and_isolation.html

If you try to remove vocal from some mp3 files the result will be “strange sounding” lo-fi grainy mp3-compression-artifacts, i.e. Audacity is doing as instructed, rather than being faulty.

All I had done was remove the vocals, and nothing else, and this random effect just messes it up!

Does this happen in Audacity before you re-export to MP3? …The “damage” done by MP3 compression does accumulate with multiple generations of MP3 compression. If you simply open an MP3 in Audacity (which de-compresses it) and then re-export to MP3 without any editing or effects, there is additional quality loss. (You may, or may-not, hear any quality loss.)

then re-downloaded it again. Import the song into Audacity, remove the vocals, and the same problem still happens!

Downloaded it from where? Are you sure it’s the same-exact download, or maybe a different file from somewhere else? i.e. If you download an MP3 for Amazon it should be the same every time.

MP3 is imperfect (lossy compression) and vocal removal is imperfect. And as Trebor says, Vocal Removal can “bring out” the imperfections in MP3 (or in digitized vinyl, etc).