The type patcher script also fixes some return statements automatically to deal with void/null differences. However, the script is a bit too eager and also patches return statements of closures inside methods:

public function something(): void
{
    $someHandler = function (): ?string {
        if (...) {
            return null;
        }
    };
}

Statements like this were patched to return;, causing a PHP fatal error.

Note: I didn't find any test for the return statement patching, so I also didn't write one for this bug. However, this bug is discovered by our own CI in #49342, so I would say we've covered the fix :)