Reply to a Hide my email does not work.

There have been some posts on this.

For me the issue was that the forwarding address (that the hide my email was forwarded to) was not a listed iCloud address but rather a gmail account that forwards to iCloud. Yes I know a silly thing to do and I am not sure how I actually set this up.

In my case to fix this is to open (in OSX) system settings, click your Apple ID, click icloud, click Hide my email.

You will be presented with a list of your hide my email's

The next bit is not obvious

Next to the + sign at the bottom of the list is kind of Mercedes symbol. Click this.

This is the change forwarding address option.

Click it and select an iCloud email account.

It makes sense why my hide my email did not work.