for future reference: many times brokers do not issue all 1099-B's by 1/31. there may be things on the form that they have to wait to finalize. in some cases the 1099-B is not issued until 3/15 and I even received a corrected 1099-B in November - well after the latest filing date.
the question becomes whether cost basis was reported to the IRS. if yes, I would wait for a letter of inquiry, if any, from the IRS. you got nothing to lose, since the loss would at worst have no effect on 2019 taxes. might even increase your refund. you could also file an amended return, but you have to wait until your return is fully processed before you could do so.
if cost basis is not reported to the IRS, then I would file an amended return
cost basis is reported if the type on the 1099-B is A or D.
