Why does `[X]` does not display? `X` would appear if I removed the brackets. Is that a bug?

```
\begin{equation}
\begin{aligned}
 [X] &= 1\\
 [Y] &= 2,
\end{aligned}
\end{equation}

```

[![enter image description here](https://i.stack.imgur.com/0zR8am.png)](https://i.stack.imgur.com/0zR8am.png)