I can’t speak for McDonalds but I can speak from other experiences with second interviews for another organisation.
When I applied for an Associate Practitioner role in the NHS, I was initially rejected and after emailing in to ask the reason why and as to how I could improve my technique, I was told that my answers were “very good”, but I didn’t answer the questions in their entirety and the advised I apply again, to which I did and passed after going through the questions again in my head after they were initially asked, also, little trick from my perspective. If you need a little more time, ask them to repeat the question as sometimes you may think of an answer whilst they’re repeating the question.
Second interviews are actually a good thing as it shows you have potential to the employer, they just need to solidify their thoughts on you.
Good luck!