Plantuml support in asciidoctor, some uml formats work, others not so much...

I felt this was the best list to ask about this, if it's not punt me in the
right direction.
I have a 13.1-RELEASE-p7 jail with all the 'latest' rubygem-asciidoctor-*
pkg's installed.
I was just starting to experiment with asciidoctor-diagram and I have
noticed that some of plantumls formats work, and some do not. Here are two
simple examples using this command line "asciidoctor -b html5 -o index.html
-r asciidoctor-diagram example.adoc -w":

plantuml that works:

[plantuml, format=svg]
....
@startuml
User -> (start)
User --> (Use the application) : Label
@enduml
....

plantuml that does not work:

[plantuml, format=svg]
....
@startuml
object London

map CapitalCity {
 UK *-> London
 USA => Washington
 Germany => Berlin
}
@enduml
....

The latter gives a syntax error. It's lifted directly from the plantuml
wesbite examples so I'm not sure that error is correct? And I know
that @startuml and @enduml *should* be optional in an asciidoc document
when using asciidoctor-diagram. But it fails even without them.

The error is:

[From string (line 4)]
@startuml
object London

map CapitalCity {
Syntax error?

Both work correctly in plantuml.com's demo server window that lets you
enter uml to render. This also happens when using other formats like yaml
or mindmap which is part of plantuml and asciidoctor-diagram is supposed to
support as well. It seems hit or miss on what formats asciidoctor-diagram
is able to render from the main plantuml.com page. Is this an issue of
asciidoctor-diagram? is it behind in supporting these formats? is it our
port? Can anyone more well versed in the asciidoctor realm explain what's
going on here?

Thank you very much!

Chris