the 1099-B does not reflect the correct tax basis because the broker does not get the k-1 and therefore can't adjust your basis for the partnership activity. All I have ever seen on the 1099-B is the original tax basis (what you paid). In the k-1 package there should be a supplemental sales schedule which should provide all the info needed to correctly report gain/loss on sale including any section 751 recapture also reported on line 20AB, if any.
another way to figure you correct tax basis is from schedule L of the k-1
capital contributed should be what you paid
+ current year income - current year loss
- line 19a withdrawals and distributions
+ the amount, if any, from 20AB
this is reported on the 1099B/8949 not in the sales section of the k-1. type B proceeds but not tax basis reported to the iRS
only the amount on 20AB,if any, is reported as ordinary income through the k-1 sales section
