I've never heard of Miracle Fruit so did some on-line searches. Very interesting! From what I've seen in pics and read on-line, the berries should be fully red when ripe. Here's a similar thread from 11 years ago: Miracle Fruit Ripening Time

The ones I've eaten have been red all over, no yellow tip. That said, since the taste is not important, ripeness may not matter much as long as the miraculin glycoprotein is present.