I have the same situation with Shire --> Takeda. The Shire / Takeda merger was a taxable exchange. My shares originated back from Baxter in 2015. Schwab just sent out "corrected" 1099 now showing basis of $0 for the Shire shares. When I went back and reviewed the transactions involved, it seems that the payment that was made on Jan 22, 2019 from the Shire merger, more than covered my basis in Shire. That resulted in $negative cost basis for the Shire shares that were exchanged for Takeda. Schwab also added another line of long term gain for the amount of the Jan 22 payment that exceeded my Shire basis.
I think $0 basis is indeed correct, at least in my case. I'm not happy however cause I now need to do amended return!
