sample6.cs

string GetAsciiString(ReadOnlySequence<byte> buffer)
{
    if (buffer.IsSingleSegment)
    {
        return Encoding.ASCII.GetString(buffer.First.Span);
    }

    return string.Create((int)buffer.Length, buffer, (span, sequence) =>
    {
        foreach (var segment in sequence)
        {
            Encoding.ASCII.GetChars(segment.Span, span);

            span = span.Slice(segment.Length);
        }
    });
}

Looks like it should be yield returning span maybe?

        foreach (var segment in sequence)
        {
            Encoding.ASCII.GetChars(segment.Span, span);

            span = span.Slice(segment.Length);
        }