Hey Musikbear, I have finally figured out the solution to my problem!
So, like I have said previously, the "distortion" (Extra loudness/muddiness of the instrument track) was actually caused by........... CALF Bass Enhancer LADSPA.
Well, at least for ME anyway.
How I found out is that the project (the one I presented to you) had only 2 tracks that got distorted. I thought it was random at first, but it wasn't. When I moved onto my other projects, and the same thing distortion happened upon some of the instrument tracks, I started taking mental notes of which instrument tracks were getting distorted after export and which weren't.
Something HAD to have been causing it, and if it wasn't my PC, it wasn't the program.. so maybe an FX? I checked the tracks getting distorted + their effects and came to the conclusion it was the Calf Bass Enhancer LADSPA effect.
NO OTHER instrument track were having issues after many trials of exportation, only those with that effect.
To test, I removed the effect (and replaced it with BassBooster). Result? On three projects I had with the problem, all of them were successful! No sudden boost of distorted loudness!
I tried around 5-6 times of exporting among the three, they were all exported correctly after the removal of Calf Bass Enhancer.
And remember that demo project you told me to try exporting? Guess why I was able to export it correctly? Yep! I checked, and NONE of the tracks had the Calf Bass Enhancer effect! Which is why everything went well with that one.
So yeah! My solution is fixed. And the cause was because of the Effect/FX: CALF Bass Enhancer LADSPA.
Now, is this weird glitch only occurring to me? Is the FX faulty, only to me, or to other people? It's strange that that's what caused all of this.
But yes, thanks for the support Musikbear, I do appreciate it!