This attribute is called caption now, not title , to be consistent across all resource types inserted as figures:
https://leanpub.com/markua/read#leanpub-auto-figures
So, you'll want to do a query-replace of title: to caption:
I recently finished the first complete draft of the Markua spec https://leanpub.com/markua/read including support for courses, and Scott recently made this change to code samples. I should have posted specifically about the title to caption change; sorry.
Finally, we now have a reasonably complete list of what is still TODO in Leanpub's Markua support here: