I've found that \lbrack, despite being \defd to [, doesn't get treated as the beginning of an optional argument in LaTeX:

\begin{array}
...
\\\lbrack 2ex \rbrack
...
\end{array}

doesn't get treated the same as ...\\[2ex] in LaTeX. However, KaTeX treats them the same (with \def\lbrack{[}). I believe this is because optional argument ([) detection uses \futurelet which grabs the next symbol, but doesn't expand it. Assuming this gets verified, we should adjust optional argument detection to use the appropriate MacroExpander tools.