I was a minority shareholder in a closely held C Corp. The business was sold in an asset sale setting rather than a stock sale. Most of the funds were distributed in 2018 and were included in a 1099-DIV Box 9 cash liquidation. The final residual funds will be distributed in 2019 with another 1099-DIV I presume.
My cost basis would be difficult to split out over the two years and I have read it is best to wait until all proceeds are received.