"Overseas ACH" isn't a thing; anything which looks like it is fronted by a US institution
>There are various ACH systems around the world. The World Bank identified 87 systems in their 2010 Survey
Seems to me like there are many different ACH systems, but perhaps the one referred to colloquially as ACH is specifically FedACH, the American system.
Giro might be the other, possibly broader term for something similar:
There are many national and international equivalents, but the linked Wikipedia page gets many of the examples pretty wrong, listing e.g. card acquirers/processors and central banks as examples for other such networks.
Direct equivalents would e.g. be SEPA Credit Transfer (but not SEPA Instant) and SEPA Direct Debit in the Eurozone or FPS (credit push only) in the UK.
https://www.nacha.org/content/international-ach-transactions...
It still Nacha which is a US institution as you mention. However, I don't see how that protects in the case of a fraudulent overseas transaction. What recourse do you have if an overseas bad actor tricks you into sending money to his overseas account via international ACH?
1) settle the transaction in the next few days (3 usually).
2) ask for additional information on the next few days.
3) deny (return) the transaction if it looks too suspicious according to the bank.
[1] https://www.nacha.org/system/files/2019-07/IAT-Specific-Data...
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.
If it surpass the 3 day window a "late return" will be sent to notify the origination that the transaction failed, however, the money may already have been transferred to the receiver.
[1]: https://www.moderntreasury.com/learn/ach-return-code-referen...