I-751 (Removal of Conditions) Extension Letter Question

I received a notice on May 16th that my expiring green card (expiring in early July) was extended for 48 months, as expected. I received the Biometrics notice and already completed it. However, I have not received the formal extension letter (with the green letterhead) that allows international travel and my lawyer (who is actually supposed to receive it) has not received it either.

I spoke to my lawyer and she said that for the last 2 months neither she or any of the other immigration attorneys she communicates with have received this original extension letter for any of their clients. She suggested we either call USCIS, wait a bit longer, or look into getting an ADIT stamp.

Has anyone here received the original extension letter recently or have any relevant experience? Is there a known delay with the extension letter?