I think I have the answer. Since Brokerage only reports proceeds and not the cost basis, you adjust cost basis on the 1099 to get your actual net gain/loss.
So, if Proshares shows phantom gains on k-1, you don't change anything on k-1 because that's what's reported (flows to Schedule D). On the Schedule D where your brokerage reports proceeds, you increase the cost basis reported by these phantom gains.
Net result is your tax return reflects your actual gain reported by your Brokerage.