I loaded up a random seed, and spawned on a taiga island. I was about to take a dip in the ocean, but saw a group of wolves attacking some sheep.

This isn't out of the ordinary, but there wasn't something right, I didn't see any sheep around me! The wolves successfully killed the "sheep," and they dropped a block of wool and some mutton as they should. However I couldn't see the sheep as if they were invisible.

I tried wandering and punching to see if I could interact with any sheep that may be near by, but to no avail, so it's possible that this issue also rendered players unable to touch them. I also restarted the game to see if it could be a rendering issue of some sort, still didn't fix the problem.

This was my first time using 1.13,  and the seed was "-4688105619867212766."

The attached file shows a screenshot of the items the sheep dropped, and the video clip (link: Minecraft 1.13 Invisible Sheep Bug) is footage of me loading up another world with the same seed. The footage shows that sheep are present by their baaa-ing, and me attempting to interact with them. You can also see the wolves killing the invisible sheep.