I believe that EAD should be filed at the office where your I-485 application resides. Since the application had been transferred to Miami, Miami should probably be the office to send the EAD application. That is perhaps why your application was transferred to Miami.
As long as it is after 90 days since you filed your EAD, you should be able to go to Miami office for IEAD. I did that before in Miami. Prepare to be there on 2 separate days. First day to get an appointment, 2nd day for EAD.