Six years ago, there was an issue (#679) where the fraction bar was not visible. This issue is similar, but it is sporadic, and only happens in Notable previews (HTML renders fine in Safari and Chrome). It happens with and without a \begin{aligned} block.

I have tested the problem on two version of MacOS. They each have the problem, but not necessarily with the same line of the 'aligned' block:

1. Sonoma 14.6 (Intel)
2. Sequoia 15.2 (M3)

Both versions are using Notable 1.8.4

Here is a screenshot of the issue:

Here is the .md code that created that image:

$$
\small
\begin{aligned}
\int_{1}^{4}\left[f(x)-g(x)\right]dx&=\left. -\frac{x^3}{3}+\frac{5x^2}{2}-4x \right|_{1}^{4} \\[1em]
                         &=-\frac{(4)^3}{3}+\frac{5(4)^2}{2}-4(4)-\left[\text{--}\frac{11}{6}\right] \\[1em]
                         &=-\frac{64}{3}+40-16+\frac{11}{6} \\[1em]
                         &=-\frac{128}{6}+24+\frac{11}{6} \\[1.05em]
                         &=-\frac{128}{6}+\frac{144}{6}+\frac{11}{6} \\[1em]
                         &=\frac{16}{6}+\frac{11}{6} \\[1.02em]
                         &=\textcolor{orange}{\boxed{\frac{9}{2}}}
\end{aligned}
$$

I use [1em] for aesthetics, but I use [1.05em] as a workaround (forces recalculation of some sort?). I can relatively easily make the fraction bars appears as they should by tinkering with the em values; however, what works for one Mac does not necessarily work for the other.

Also, for single-line blocks that do not use aligned, the workaround is to put the line in a \begin{aligned}; such as,

$$
\begin{aligned}
r = \frac{C}{\pi} \\[-.01em]
\end{aligned}
$$