I don't see the evidence of this here, it's just an Amazon page.
I dont know how I'd post evidence. I can explain how to recreate. I had a kindle device for a long time. I recently reset my device to factory. Then reattached the kindle to my account by booting up the kindle device and logging in with my Amazon credentials. Next, I went to my settings page to see what my "Send to Kindle" email address was and it was my "<AmazonPassword>@kindle.com" without the numbers and special characters.
Then do that, because it might be a bug or even intended, but there's no proof in this post, there's not even context from which we ourselves could investigate. If that's actually happening and you didn't accidentally type your password in the wrong box, then it'll be a big deal, but for now this is useless, sorry friend.
I just put the context in the reply above. I'm a (systemNetwork/admin/engineer) by trade and hobby, I'm not perfect, and I definitely make mistakes but I guarantee you I didn't put anything in the "wrong box". It was pretty straight forward.
Step 1. Turn on new Kindle device.
Step 2. Log into Kindle Device with Amazon account.
Step 3. Go to settings, the "your Account"
Step 4. Look at the last line and it shows your passwword(partially) in plain text.