I'm sure you agree that 'up' is the opposite of 'down' and that all things fall 'down'. What you seem to (quite amazingly) be missing is that on a Real Round Earth the terms up and down are relative not absolute. We are quite aware that in FE up is an absolute term, but to use that fact as a prove of flat Earth means you are saying:
1. If the Earth is flat, 'up' is an absolute direction.
2. Bubbles should always go 'up'
3. Bubbles do always rise from the ground, going apparently 'up'
4. This 'up' is absolute, and therefore the same direction everywhere.
5. On a Round Earth this would lead to bubbles going into the ground.
6. Bubbles don't go into the ground.
7. Round earth is false.
But your fault is in point 4, where you have assumed what you are trying to prove, reducing your 'proof' to:
"The Earth is flat, therefore the Earth is flat."
Compelling in its simplicity, but a sad logical fallacy.
Edit: That last line has become my new poem about Flat Earth. It's called 'Narcproof'.