Phase inversion isn't going to show up as Left being Right or Right being Left. Phase doesn't really affect anything until you mix it with tracks recorded with the opposite phase. You can get this effect with differences in distance of the mic(s) from the source. Or because the clocks on different devices (even of the same make / model) differ. i.e. using 2x 2 track recorders instead of 1x 4 track recorder. Not saying that that is what's going on. Basically if you invert the track and it corrects, that's a good indicator. Or if it's different by itself, than it is mixed, which you seemed to have already noticed. Could be lots of things, but phase is a common thing when dealing with mixes. If it's a difference in clocks, it'll come and go in and out of phase as things line up, then get out of alignment. Stuff which wasn't present in the original source, or present in the individual tracks when solo'd.