\starttext

\startformula

  \startmathalignment[m=2,distance=4em]

   \NC \text{slope}_A \NC = 0

   \NC \text{slope}_B \NC = \frac{1}{2} \NR

  \stopmathalignment

\stopformula

\stoptext

In ConTeXt 2017.05.15 (shipped with TexLive 2017), the second column is formatted as text rather than math. The result gives ‘slope_B’ rather than placing B as a subscript.

Online and with 2016.05.17 (shipped with TexLive 2016), the second column is formatted correctly as math.