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.
https://www.in.gov/dor/reference/files/ib98.pdf
