AppleInsider can confirm that it’s pretty easy to get somebody’s real email from Hide My Email, but it’s also not much of a concern. If you’re using the feature as intended as a spam filter, there’s really no issue at all.
Hide My Email is supposed to do just that, but it doesn’t always
The Hide My Email feature allows iPhone, Mac, and iPad owners to sign up for websites and services using an anonymous email address. All emails sent to that address are then invisibly forwarded to the user’s real email address, preserving their privacy.
However, it has been reported that the feature can be bypassed, allowing someone to see a user’s real address. Apple is yet to fix the issue, despite having known about it for more than a year.