First off, the BBCode to HTML is done by vB itself, not my plugin. I didn't want to reinvent the wheel. So if this was actually a bug, it'd be a vB one.
Second, it's not a bug, it's actually intended behavior:
Code:
some text underlined text
If you don't want something to parse, then use the [ noparse ] tag.
Code:
some text [u]underlined text[/u]
What you posted only works here because this site doesn't have my plugin installed and therefore doesn't know about / parse the [youtube] tag.