caption label(Figure) and automatic number is missing.
There is no problem with just an image.
And there is no problem with v1.5.5.
I have also set ":figure-caption:" to a value.
This is a known regression in Asciidoctor Diagram caused by a change to core. It's now being addressed.
Since the release of Asciidoctor 1.5.6.1, we've configured the Asciidoctor Diagram test suite to run against upstream core. That should allow us to avoid these types of regressions in the future (before a version of core gets released).
Everyone, thank you for your correspondence.