doks: KaTex cannot render some equations

The problem is that when I use KaTex to render my equations, some eqautions can be rendered while others cannot be. Such equation like below cannot be rendered: (use env aligned)

$$\left\{
\begin{aligned}
PDE \quad &u_{tt}-a^2u_{xx}=0 \\
B.C.\quad &u(0,t)=u(l,t)=0\quad t\ge 0 \\
I.C.\quad & u(x,0)=\varphi(x),u_t(x,0)=\phi(x)\quad 0\le x\le l
\end{aligned}
\right.$$

And the result is below:

Thanks for your time !