There is no way to prove that the earth isn't actually flat but every observation conspires to make it look round. For instance some flat earthers say that the atmosphere reflects light in the exact way that makes it look round.
Take any phenomena on a globe earth, describe the exact same thing in flat earth coordinates and then say that everything weird in the equations is a new physical effect you just discovered.
I don't believe I've seen a flat earther explanation of foucalt's pendulum yet, but perhaps they have one
That's a void argument.
If every observation conspires to make it look round, it's round because observation is all we have. Refusing to accept observational evidence that forms a coherent explanation is either anti-science or anti-definition-of-words. This justification for flat earth exits the realm of scientific inquiry and enters the realm of Cartesian evil demons, a hypothesis even Descartes rejected.
> describe the exact same thing in flat earth coordinates and then say that everything weird in the equations is a new physical effect you just discovered
…which have other consequences that are easily disproven.
Flat earthers are empirical cosplayers. It mostly seems they just want something to argue about and couldn’t come up with anything original.