Obviously, I have no insight as to how this situation came about, exactly, but it is virtually irrelevant since any adjustment would have to be made in Forms Mode directly on Form 8960 and the original poster is, from the profile, using an online version of TurboTax (in which the adjustment is not supported).
See https://digitalasset.intuit.com/DOCUMENT/A7aTcVUwu/2018-unsupported-calcs-L1FQfTTgn-073019.pdf
