Why don't multi-line tweets show up properly on the Twitter homepage?
Unfortunately, the Twitter homepage does not display line breaks (new lines) – it prints everything as a single line.
This means that if your Unicode art spans multiple lines, it will not display as intended.
This is a bug with the Twitter homepage and it has been reported to Twitter. Once they fix this bug, multi-line tweets will display as intended on the Twitter homepage.
In the meanwhile, your multi-line tweets will show up properly in other Twitter clients like Tweetie, Kiwi, and Tweetdeck.