In this answer the underline below $k$ in the exponent in the definition of $\binom nk$ cannot be seen. It took \displaystyle
for it to appear.
I can reproduce the issue here as well. Compare this: $\binom nk = \frac{n^\underline{k}}{k!}$, with this: $$\binom nk = \frac{n^\underline{k}}{k!}$$
The code is the same,
\binom nk = \frac{n^\underline{k}}{k!}
but in the first instance the underline is not appearing. For reference, this is what I'm seeing:
My browser is Chromium 55.0.2883.87 (64-bit), and other users reported the same issue.