Is it intentional that you cannot skip lines when using optional environment parameters with LaTeX3's `xparse`?

It seems that you can skip lines with mandatory parameters.

Compare
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```
\documentclass{article}
\usepackage{fontspec}
\usepackage{xparse}

\NewDocumentEnvironment{mandatory}{ m m m }
{\section{Mandatory}}
{#1\par #2\par #3}

\NewDocumentEnvironment{optional}{ O{first} O{second} O{third} }
{\section{Optional}}
{#1\par #2\par #3}

\begin{document}

\begin{mandatory}
{FIRST}
{SECOND}
{THIRD}
\end{mandatory}

\begin{optional} %<- Here is the problem child.
[FIRST]
[SECOND]
[THIRD]
\end{optional}

\begin{optional}[FIRST][SECOND][THIRD]
\end{optional}

\end{document}

```

Output
======

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