I've resolved my own question. California provides an ACA subsidy in addition to the federal one. The California subsidy is targeted towards those eligible for the federal subsidy who have higher incomes and therefore receive less of the federal subsidy. Reducing my AGI with the unemployment exclusion brought my income below the California threshold to receive the California ACA subsidy which I had been receiving during the year. As a result, I now have to pay back the California ACA subsidy, hence the increase in my California state taxes as a result of the unemployment exclusion.
