It looks like TurboTax programming correctly reflects the current law.
According to the instructions for Schedule IN-529: Indiana’s College Choice 529 Education Savings Plan Credit, "This credit … is available to an individual filing a single return or to a married couple filing a joint return."
The document is only available in pdf, so I can't post a direct link here, but you can access it from 2018 INDIVIDUAL INCOME TAX FORMS; open the "IN-529" link to find the form and its instructions.
I encountered the same issue. Wife and I file MFS due to her income-based repayment on student loans. It was nice to double-dip on the 529 credit because the 529 credit was per taxpayer. Now, when they changed the law in 2018 to allow 529 to be used for K-12 they also eliminated the credit entirely for people who file MFS.
Here is a bulletin from the Indiana DoR.
Bummer! It would be nice to know IND rationale for excluding MFS filers from receiving this credit. If it was a concern with 'double dipping' on the credit (married filing separately could claim 2x?) I would think they (INDIANA) could limit to only one credit due per related contribution.