De-tailing a Miku forcefully is harmful as they use it as a balancer during dancing and as a feelers for possible unseen obstacles. But on rare occasions they might shed their tails themselves. Don't be alarmed as this is perfectly healthy and normal. They do it as a sign to show that they have reaffirmed themselves to conquer the challenges that they have. Just be sure to help them as losing their tails can make them more spontaneous. And remember that it's not the hair or outfit that makes a Miku, it's the heart 🩵