All you need to do is die and you can see mobs underground or on the ground. This got to be a bug because I did not see this in the game before. If you have stuff in your inventory, as SirChristopher19 (username) say, it will look like the stuff fall out of the world when you die when it don't. I tried it out in the Nether and End dimensions too to see if it happen there too, it do. I also added two debug reports. They both are maybe the same but I don't know. Maybe the reports will help to figure out what is wrong.
I am not going to fight over this matter, I still see it. I will be happy when this bug is fixed because it been around since 1.14. I want to be done with this bug.
I've noticed this as well in my 1.14 survival world. When I fell off a tall tower in my world, I could see through everything and even saw my inventory items fall out of the world, but they were all back at the tower when I respawned.