re: "Incomplete" Cost Basis

It happened to me with TD Ameritrade. I sent them a message (through my app / account services) and told them my cost basis which I had known by looking up a previous statement from my brokerage prior to TDA. They had it fixed within a day. Never even asked for corroboration on the cost basis. I could have made it up (given a higher cost basis) to limit my taxable liability.