What if an overseas bank just claims they didn't receive your transfer when they did?

Others have covered the overseas part, but for the bank claims they didn't receive you're transfer part, I would expect that to show up on the settlement process. If there is a disagreement between banks, when they later check how much each bank owes each other, there should be a discrepancy the banks need to work out.

Note: not an expert in how ACH works or how settlement is done.