...except when it affects the documented behavior of functions like lc in certain situations.

As I previously mentioned, lc is (intentionally) buggy (for backwards compatibility) when not using the unicode_strings feature. So yes, it makes sense to document it.

In what sense is this not an array being returned:

In every sense. Four scalars are returned by split, which are then assigned to an existing array.

..yet sometimes does

No. split is a function (named operator), not a sub.

But while I won't rule out the possibility of a change for subs, it's definitely not possible for a function. "Returning" scalars from a function definitely has no effect on their internal storage format. That post shows no evidence that returning a value had any effect on it whatsover. The returned scalars are as the function created them.