Answered by Shaykh Faraz Rabbani
Does drawing blood for medical reasons break one’s fast?
No, it does not break one’s fast. It is only disliked if it could weaken one from being able to fast.
Does forgetting to break it at the right time invalidate one’s fast?
If you MISTAKENLY break your fast before Maghrib actually came in, your fast is broken.
Mistakes break the fast, though, if excusable, are not sinful. Forgetfully doing that which breaks the fast does not break the fast.
sorry, I was unclear. the question concerns forgetting to break one’s fast at the right time, and ending up breaking it 5 hours after maghrib has come in.
If you mean by ‘ forgetting to break it at the right time’ that the time of Iftar passed and you did eat or drink anything then that does not invalidate the fast. The fast is automatically broken when the time of Iftar sets it even if you did not eat anything.