It's annoying because if the network goes out after you click the Post button, then not only will the message and the subject be erased, but also you can't even use the back button to go back to the editing page, because that's gone too.
I lost a few PM drafts this way.
If I've navigated away it doesn't usually remove the text so I've gone through the same situation you've discussed. Sometimes the text's removed. I don't know why going forward or back in my browser nav buttons isn't consistent each time but I'm happy it's there most of the time so I don't have to retype all of my post.
Using the back and forward buttons should work, but a page reload invariably discards the page data.