After cooking chili peppers lose some heat, but I've never personally experienced them losing heat during storage.
You didn't mention if you experienced the same results as your friends. Have you opened a sealed package after 24 hours and tasted them. If not it could just be your friend's own subjective taste preference. If you get the opportunity, have one or more of your friends taste them within the first 24 hours of when they are done, and before packaging to see what they say. Then have them report back, after they have opened a sealed package from the same batch.