The accounting for the convertible was convoluted. Especially due to the stupid bull call spread Tesla bought in conjunction with the deal. The 10-K says:
In accordance with GAAP relating to embedded conversion features, we initially valued and bifurcated the conversion features associated with these notes. We recorded to stockholders’ equity $188.1 million for the 2019 Notes’ conversion feature and $369.4 million for the 2021 Notes’ conversion feature. The resulting debt discounts are being amortized to interest expense at an effective interest rate of 4.89% and 5.96%, respectively.
You can think of it as a reversal of the 188.1m of non-cash interest they expensed over the 5 years of the bond's life. It ate away at the 188.1m of equity, but on 3/1 when nobody converted that equity "reappeared" and created a non-cash profit. They subtract that non-cash profit when calculating OCF.
At least that's my first take, I need to spend more time on it to be sure.