Now this is interesting! Here is a facet list from a cube:

        usemtl right
        f 1/1 2/2 4/3 3/4
        usemtl top
        f 3/5 4/6 8/7 7/8
        usemtl bottom
        f 5/9 6/10 2/11 1/12
        usemtl left
        f 5/13 7/14 8/15 6/16
        usemtl front
        f 2/17 6/18 8/19 4/20
        usemtl back
        f 1/21 3/22 7/23 5/24

The right face is at the top of the list, and that is the face that gets the wrong colour. Below the order of the order of the usemtl blocks has been edited so that the top face is now at the top of the list:

        usemtl top
        f 3/5 4/6 8/7 7/8
        usemtl right
        f 1/1 2/2 4/3 3/4
        usemtl bottom
        f 5/9 6/10 2/11 1/12
        usemtl left
        f 5/13 7/14 8/15 6/16
        usemtl front
        f 2/17 6/18 8/19 4/20
        usemtl back
        f 1/21 3/22 7/23 5/24

Can you guess which face now displays the wrong colour? That correct, it's the top face that is now wrong. What about this one?

        usemtl bottom
        f 5/9 6/10 2/11 1/12
        usemtl top
        f 3/5 4/6 8/7 7/8
        usemtl right
        f 1/1 2/2 4/3 3/4
        usemtl left
        f 5/13 7/14 8/15 6/16
        usemtl front
        f 2/17 6/18 8/19 4/20
        usemtl back
        f 1/21 3/22 7/23 5/24

You guessed it! It is the bottom face that now displays the wrong colour. It seems that whatever face is at the top of the list, will be the one with the wrong colour. No other parts of the geometry were changed, only the order of the usemtl blocks.