Predboy, yes they did. Look at when the first one roars after the helm is removed. Or how the other is almsot permanently showing them wide apart when it's advancing on its fallen prey in the slaughterhouse place. SiL: Yes, it's
vaguely there when I've seen it in produciton stills, but never anywhere else and yes, I have tried.
I put it in the same category as seeing the foot when Frost gets incinerated and falls down the shaft, the rest of Bishop's body in the hole, or the apparent change in the number of fingers the creature has or the third film switching between different configurations of legs. It's there, but nobody notices enough for it to matter in any way.