I created an environment where I override the & character to be active
and use it as a macro (rather than an alignment character). I am now
running into the issue, that theoretically the user might not pass an
optional argument and also use the & token as the first character
inside the environment. In that case the token is interpreted as an
alignment character.
The environment in question is currently defined with:
\NewDocumentEnvironment { sdapsarray } { o }
Now, this is a very rare corner case for me, and I am happy to simply
document the limitation. But, I do wonder whether this may be
considered a bug in the optional argument parser.