Math block works with ```math but not $$

[flokno]

I have the following snippet:

$$ \begin{equation} \begin{pmatrix} m_1 \ddot{\mathbf{u}}1 \ m_2 \ddot{\mathbf{u}}2 \ \vdots \ m{N_a} \ddot{\mathbf{u}}{N_a} \end{pmatrix}= \begin{pmatrix} \mathbf{\Phi}{11} & \mathbf{\Phi}{12} & \cdots & \mathbf{\Phi}{1N_a} \ \mathbf{\Phi}{21} & \mathbf{\Phi}{22} & \cdots & \mathbf{\Phi}{2N_a} \ \vdots & \vdots & \ddots & \vdots \ \mathbf{\Phi}{N_a1} & \mathbf{\Phi}{N_a2} & \cdots & \mathbf{\Phi}_{N_aN_a} \end{pmatrix} \begin{pmatrix} \mathbf{u}_1\ \mathbf{u}2 \ \vdots \ \mathbf{u}{N_a} \ \end{pmatrix} \end{equation} $$

which renders fine when using the ```math syntax:

\begin{equation}
\begin{pmatrix}
m_1 \ddot{\mathbf{u}}_1 \\
m_2 \ddot{\mathbf{u}}_2 \\
\vdots \\
m_{N_a} \ddot{\mathbf{u}}_{N_a}
\end{pmatrix}=
\begin{pmatrix}
\mathbf{\Phi}_{11} & \mathbf{\Phi}_{12} & \cdots & \mathbf{\Phi}_{1N_a} \\
\mathbf{\Phi}_{21} & \mathbf{\Phi}_{22} & \cdots & \mathbf{\Phi}_{2N_a} \\
\vdots & \vdots & \ddots & \vdots \\
\mathbf{\Phi}_{N_a1} & \mathbf{\Phi}_{N_a2} & \cdots & \mathbf{\Phi}_{N_aN_a}
\end{pmatrix}
\begin{pmatrix}
\mathbf{u}_1\\
\mathbf{u}_2 \\
\vdots \\
\mathbf{u}_{N_a} \\
\end{pmatrix}
\end{equation}

Am I doing something wrong when using $$?

[flokno]

A more minimal example. It seems that \ddot triggers the error.

With $$:

$$ \ddot{a}{a_b} a{b} $$

With ```math:

\ddot{a}_{a_b}
a_{b}

[flokno]

Not \ddot, but probably the combination of \ and too many _:

$$ \mathbf{a}{b{c_d}} $$

With ```math:

\mathbf{a}_{b_{c_d}}