This can be reenacted when a zombie is chasing you and you run inside either entrance of the supermarket.

Found I can reproduce this for my Player Character:
@ western most of the two supermarkets in the western Cherno-Balota High Rise Buildings.
Approach the supermarket from the back, use the vault/hop key/function to mount the plinth at the back of the supermarket. Instead of climbing it, the player is placed in the (world) ground level. Loot/Items can still be gathered in this condition, and weapons can be successfully fired at zombies.

What's more, I once punched a zombie while inside a building and it actually fell to the ground level. It could still run around, but couldn't attack me.

Can vault into supermarket and hide under the floor space. Can loot without being seen, and pop up out of floor to shoot/attack, then prone back into the floor. Zombies can/always enter the building in this manner, although they do not prone.

When Zombies run into supermarkets/grocery store they glitch into the floor. Their legs are inside the floor, while their upper torso remains above the floor.

Can also recreate this as a player by vaulting into the supermarket. You can then act as the zombies do above, or even go prone to make yourelf invisible underneath the floor. You can still loot items, and pop up out of the floor to shoot/attack.

Real immersion breaker and potential annoyance here.