I’m not religious but there’s a significant difference here.
Burden of proof is on the person making the assertion in both cases, but we can’t prove without a doubt that god doesn’t exist even if we don’t feel there’s enough evidence to suggest he is. There is, however, concrete evidence the earth isn’t flat, so no matter who the burden is on it’s demonstrably false.
Put another way: You can concretely observe without a doubt that not only is the earth not flat, but also that it can’t be flat. We can’t confidently say god can’t exist.
I don't think you've thought through what you're trying to assert. A god could make you believe anything they wanted to about the earth. So if you cannot disprove a god, then you cannot disprove the theory that the earth is flat.
and most people who believe in God will cite some evidence - religious experiences, or philosophical proofs or whatever. Whether you accept that evidence is sufficient or not, it is in an entirely different class.