Bug introduced in rP25210.
The previous Unit code had logic to 'fix' variants, by picking them initially. The new code removed that, as I completely missed its purpose. I sorta went with the idea that the random selection would return the same results since it's the same RNG, but of course if the layout changes (more/fewer variants/groups/...) in a given prop then that suddenly doesn't work anymore.
The solution is to re-introduce the 'freezing' of actor selections on CUnit creation. This makes sure that units don't change their purely-aesthetic selections when e.g. animations change.
See the horse-changing-when-running issue.
Reported by: wowgetoffyourcellphone