I found this seal washed up, what's happened to it's head?

Apologies if not the right place to post this. I don't really know anything about marine things. Thought this was interesting.

Anyway it seems like it's head is severely decomposed/almost melted? Can't even really make out any features. Whereas the rest of it looks like it has only just died. Curious on why this may be.

Of course our theory is navy supersonic weapons testing lmao but I'm sure it's something more typical about decomposition or something that would specifically eat that area and leave the blubber.