Very likely. I doubt the preamble has been handled / customized for the deck.js backend. Would you like to give it a try? Perhaps start by filling an issue in the asciidoctor-backends repo.
One way to address this is to create a dedicated slide for the preamble (wrap it in the correct HTML element). Another solution is to output nothing in the block_preamble.html.haml file and instead check for the presence of a preamble in the document.html.haml and weave it into the title slide or something.