Unable to download Apple Developer keys after initial creation

In my Apple developer account I have created a key for use with APNs across all my apps. On creation it allows me to download it once I have finished. After that the 'download' button is greyed-out and I can't download it again. Presumably this is by design.

Can anyone tell me why it doesn't allow a second download like you can for the certificates/profiles?

You can't re-download the IOS Push Notification Auth key. Only public key is retained by Apple and a private key which you retain and protect.

See documentation section Provider-to-APNs Connection Trust.

Token-based provider connection trust: A provider using the HTTP/2-based API can use JSON web tokens (JWT) to provide validation credentials for connection with APNs. In this scheme, you provision a public key to be retained by Apple, and a private key which you retain and protect.

You can see warning too when you create an Auth Key.

Download and Back Up After downloading your key, it cannot be re-downloaded as the server copy is removed. If you are not prepared to download your key at this time, click Done and download it at a later time. Be sure to save a backup of your key in a secure place.