Information disclosure vulnerability when using a ipv6 address to login

Steps to reproduce:

1. Use an ipv6 internet connection
2. Navigate to login screen
3. Use a valid username with a wrong password
4. Click "Sign in"

Result:
See screenshot...

This in fact an information disclosure vulnerability, since you can search for existing user accounts that way. I asked for an alternative way to submit this issue, but didn't get an answer for about two weeks.