It is almost certain that this evolving risk was disclosed to Verizon (and other officially interested parties) during the initial stages of the sale negotiatin

It is almost certain that this evolving risk was disclosed to Verizon (and other officially interested parties) during the initial stages of the sale negotiating process, and could well explain the difficult gestation of the Yahoo sale. Note that standard procedure would have been for interested parties wishing to enter into formal discussions, to sign watertight non-disclosures at the risk of very big legal liabilities if they don't abide by them. Any M&A professional would know that the valuation hit of "owning up early" is much smaller than the catastrophic effect of trying to hide such a thing until the inevitable noisy leak.