Why is WhatsApp microphone not turning blue when listening to voice messages on iPhone?

It is a bug in WhatsApp that has been reported here. Unfortunately, Meta does not provide a Support Forum where their users can discuss the issues with their app and they are not likely to get that information from here either. They certainly do not make it easy for their customers to report problems to them, but it will take an update in WhatsApp to resolve this bug. The app has been updated 12 times in just the last month and any one of them could have introduced that bug. The last update was 3 hours ago, so you can try that one, but from the last report I saw, I believe another user tried it and it still did not work.