Zee Fern

This is normal behavior for blocks unless it is certain gravity-affected blocks like sand, gravel, snow layer, etc.

I was walking on snowy terrain (It might have been snow and sand) with my horse (only having a saddle with no armor/name tag) and my dog (name tagged). Suddenly, my horse froze and started glitching and disappearing. Another wolf nearby was also glitching and disappearing. My dog disappeared mid-bark. 

I was bound to the horse and then it froze and had to restart my game. 

I'm not sure if this is related, but since it involves disappearing tamed mobs, I figured it was good information.

However, the other times this has happened, I was not on a horse nor on snowy biomes. Those were the times I just logged on and everything was empty.

This also happened to me! But my tamed dog disappeared too. It's happened before when I log on to Minecraft, and made me so sad when all of my dogs disappeared 😞