Why is that? If you use the code buttons for all your markup, the close tags system works perfectly. If you don't want the close tags functionality, then use normal mode or type your tags in the old, long-hand fashion.

The only way I could account for users deleting tags they entered with the code buttons would be to write some very nasty regular expressions to parse the message textarea, which would not be pleasant to write in Javascript, and is not something I plan to do right now.