I have always declined to provide copies to any subpoena request.
You keep right on doing that. This is different.
In essence, you made the SAR available to several law enforcement agencies when you clicked "submit." This is just one more agency. It's unusual that they would ask you for it rather than query the data base, but you apparently notified your regulator (as well as FinCEN) of the subpoena just as you are required by law and regulation to do. The former has approved the disclosure. This additional disclosure is just one more slice off an already cut loaf. Your attorney should still be involved, but it's difficult to imagine what his or her objections could be.
You have maintained the legal confidentiality of the document.
Now, it's up to law enforcement to maintain the document's confidentiality. Your SAR is not "evidence:" it cannot be offered as proof of anything. It's the supporting documentation that may be entered into evidence in a courtroom and become public. Take comfort in the fact that it's a crime for law enforcement to disclose the existence of a SAR; they have just as much incentive to be careful as your bank does. In this specific instance, as noted, this is a grand jury proceeding; everything about it is confidential and there will be no public record of the proceedings.