Taxpayer (TP) in question may be in a similar situation, as presented in original question.

TP’s  accountant correctly entered a negative number on Form 6251, Line 17 to account for a difference in cost basis for a stock sale in 2017 (exercise/AMT trigger was in 2014), but, neglected to enter a similar negative amount on Form P(540), Line 9 to account for the same difference in cost basis on the CA state return. I believe that resulted in premature usage of -all- of TP's State AMT credit (which resulted from the exercise event/AMT trigger in 2014 when taxpayer exercised the ISOs).