If path effects are used with math text, then the math text is rendered as plain text, i.e., TeX expressions are rendered verbatim. For example, the test script:

#!/usr/bin/env python
from matplotlib import pyplot as plt
from matplotlib import text, patheffects
plt.title('foo^\mathrm{bar}',
    path_effects=[patheffects.withStroke(linewidth=1, foreground='r')])
plt.savefig('test.png')

results in this image:

However, in earlier versions (v1.2.1) the text is rendered correctly as math text:

This issue was not present in v1.2.1. git bisect reveals that 4c61dc7 is the first bad commit.