In my experience the IRS is extremely slow at posting and matching corrected W-2's, 1099's etc. Sometimes this causes a mismatch letter, other times it doesn't.

The extra brokerage account is happening more and more as clients seem to have 2-10-20 brokerage accounts for whatever unknown reason.

I typically add with the reason for the amended return the brokerage house name and account number of the new account, or the name and EIN of the new K-1 or similar.