4 x4 bmatrix has broken brackets. Is this normal?

I am documenting some matrix math using Python/Sphinx. I am trying my hardest to keep it simple and use up-to-date versions of software and default values supplied by Sphinx etc.. The HTML page created by Sphinx uses MathJax to create a  4×4 bmatrix  array. However the 4×4 matrix is displayed on the web page with incomplete brackets that cover the top and bottom corners of the matrix but not the middle sections. If I cut and paste the “MathJax as TeX Commands” into a regular tex file and compile with TexStudio I get the brackets displayed properly with a complete middle section

Is it proper expected, behavior for MathJax to display broken brackets for a 4×4 matrix?  I understand this may not be an actual MathJax issue, e.g. it may be a Sphinx problem incorrectly configuring MathJax. But its difficult to know where to start and the MathJax community seems like the right place to start.

Here are the “MathJax as TeX Commands” cut from the web page display,

\begin{split}\mathbf{R} =
\begin{bmatrix}
1 & 0 & 0 & 0 \\
0 & -\cos\:2\alpha  & -\sin\:2\alpha & 0 \\
0 & +\sin\:2\alpha & +\cos\:2\alpha  &  0\\
0 & 0 & 0 & 1
\end{bmatrix}\end{split}

Thanks for any help or guidance.