What is the ruling on the fast of one who finds blood in their throat upon waking, tastes it, sometimes spits it out and sometimes does not, with this condition recurring on days of fasting and making up missed fasts?

If a fasting person tastes blood in their mouth, they should spit it out. However, if the taste of blood is constant, usually due to gum disease, then its effect is excused, and the fast is not invalidated by it. Nor is it obligatory to rinse the mouth from it, and this does not affect one's fasting.