I’ve already tried all the steps you mentioned (including checking the keys, the Gateway Error Log, and replicating the request in /IWFND/GW_CLIENT). I could also see that the request was reaching CHANGESET_PROCESS, and I even attempted to handle the logic there, but it still didn’t solve the issue.

What finally worked was disabling batch requests by setting:

oODataModel.setUseBatch(false);

This way the request is sent as a plain POST instead of being grouped in a batch with PUT or MERGE. While this is more of a workaround than a real fix, it allowed the service to process the request correctly.