Antisamy is producing the correct result - the badly formed tag(s) are removed leaving plain text alert('xss');
.
Consider the following
<b<i>>Hello World!</b</i>>
A bold and italic tag have somehow become muddled - antisamy correctly strips the broken tags leaving the text Hello World!
which is correct. That there is a plain text that looks like javascript remaining in your original test is of no concern - the harmful <script>
tag has been removed.