If the option is checked, your e-mail should not show. Maybe with those that are friendly, you could ask. IF the system is broke, eBay needs to know it ad fix it.
eBay has said that they are going to take away that option all together and hide all e-mails. Maybe that is why, it's broke.
I get about 30% of my e-mails that have the address blocked, so I know it works some times but....
If you want to get the nasty e-mails back on the system, copy and paste it as a reference when responding THROUGh the eBay system. Make sure you save the original so if eBay ever asks, you can show them.
Before I do that, I would probably block them as a buyer and explain in your e-mail that you do not wish to continue the discussion if it is going to be unnecessarily hateful. Most confrontational buyers quiet down with a calm response, but I do not know the specifics of your communications, so.....