what I expected:
Throw an splash potion of invisibility at a cave spider and only see those glowing eyes.

What actually happened:
Threw the splash potion at the cave spider and I saw its eyes, however when I flew up high I could see an opaque outline of it.

I know the visbleness of certain mob features as I have previously searched to see if my bug existed ( wool on sheep, cave spiders eyes, ect) however I'm not sure This is normal

(Also happens with Spiders, but I have checked all other mobs and does not work with them, Only cave spiders, and spiders)

The first picture below is of the newly made invisible cave spider and sure enough you can still see the glowing eyes, however the second one is as I have flown up high and now you can see the opaque outline of the cave spider. I'm not if this does work on SMP Creative/survival Or SSP Survival