Deltoids

To me, this feature is the most strange

Something is missing from that foot. I didn't notice until now.

I also missed that foot.  Not sure if it's rendering error since it looks like the other rear foot has a similar void.

I think I've noticed this in the thread posted with the close up of the rear end of the Alien on the outside of the ship.  I assumed they were like tendons that go from the heel up the back of the calf, but thought it was odd that it is exposed instead of being covered in skin.  You don't really see many voids in body structure in the wild. Would be nasty to get that caught on something while climbing or running.

I can only assume we need to look at the Organism as we would a Human who has had the outer Skin Layer removed...

The Hole in the foot is like between where the Archilles Tendon and Peroneal Retinaculum/Peroneus are.  But then the Anatomy of the Xenomorph could be different so it has another lower kind of Fibula and Tibia.

I think we also have to consider this is Sci-Fi and indeed they maybe never 100% stuck to what Anatomically would work in real life, there is a element of Fiction here, and we only need to look at the Actual Alien 1979 Xenomorph and we can ask many Questions about how this could/could-not function as a Organism.