I am not sure I understand what you mean by remote validation, but in case you are using an server calls in a custom validation rules, then I suggest using synchronous instead of asynchronous calls. That would not allow the validation function to finish before the request's response is received. Let me know if that is not what you mean and if possible - share some code snippets, so we can investigate.
Sending null if no change was made is expected in that case, because otherwise the underlying dataItem (which is not present in server binding) will not be updated.
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