Sequence Diagram: Cannot have two end of Arrow having different style

Question

Hello,

Base on reference guide section 1.7, learnt for different type of arrow styles, I found following issue:

Bob <<-> Alice

In my understand, it should have the left arrow with Thin Head and right arrow with Normal Head, however, the diagram generated shows Thin Head on both end. If there any hint on why it like this?

Thanks, Xiaoqi