What is the difference between the dislike of using a miswak for a fasting person and toothpaste, even though the taste of the toothpaste disappears?

Toothpaste does not invalidate the fast unless something reaches the stomach. The lingering taste in the mouth does no harm as long as none of its particles reach the throat. Using a siwak is recommended for a fasting person at all times, both before and after Zawal (midday), contrary to those who disliked it after Zawal. The correct view, supported by texts, is that the siwak is "a purifier for the mouth and a pleasure for the Lord."