During my playthrough of the new nether update and the addition of striders, I was excited to jump right into the new content. However, upon my friend joining my SMP and dying (to me),

he said that he couldn't move and that he could hit himself. We investigated further and it turned out to be a very strange softlock. He could break 1-tick-blocks like slime blocks and torches and redstone, but he could not break dirt. He could not take damage from lava or bows, and I could not hit him. Video linked below. The issue was only solved by reverting to the previous snapshot.

https://youtu.be/3JYDvAQ3tTA