You'll have to excuse me for being out of the routine for
years now, but I don't think it was ever necessary to compact to a new .mix. I'm not sure how I did it, but I think just saving stuff in the RA2 directory worked, with the stuff in the directory overriding the data in the .mix files.
Perhaps doing just this is worth a try? Isolating the problem is the first step in solving it. You could see if your tank works with regular Apocalypse voxels and if an Apocalypse works with you httank art.
I used to know what would at least be impossible to be the problem

.