Even when a block existed in the previous server tick and exists in the current server tick, it's still possible to fall through it.

To reproduce:

• Place a repeating command block with this command: execute at @p run fill ~-2 ~-1 ~-2 ~2 ~-1 ~2 bedrock
• Stand on a block without any part of your hitbox overlapping another block
• Break the block below you
  Result: Sometimes nothing happens, sometimes you fall through the block, even though it existed at all times server-side.

This probably happens because the client starts moving downwards a bit before receiving the message from the server on the next tick that the block is still there, but then the server simply believes that the client was right in moving downwards. This probably also means that a modded client could simply go through any block downwards, potentially even in all directions.