strange counting in Asciidoctor?

Question

I want to count my figures automatically

I use Asciidcotor 1.5.6

:desc-image-fr: FR Text
[id="image-fr",reftext="{figure-caption} {counter:figures}"]
.{desc-image-fr}
image::fr.png[align="center"]

:desc-image-ab: AB Text
.{desc-image-ab}
[plantuml,id="image-ab",format="png",title="{desc-image-ab}" reftext="{figure-caption} {counter:figures}",align="center"]
----
() "FR"          as GS
() "AB"          as AB
----
look in <<image-ab>> and <<image-fr>>

This is the result:

...here is the image...

Figure 3. FR Text

...here is rendered png of plantuml...

Figure 5. AB Text

look in Figure 4 and Figure 3

I am surprised about the "Figure 5"... Why 5 and not 4?

---

EDIT: added info from pom.xml

        <plugin>
            <groupId>org.asciidoctor</groupId>
            <artifactId>asciidoctor-maven-plugin</artifactId>
            <version>1.5.6</version>

            <dependencies>
                <dependency>
                    <groupId>org.jruby</groupId>
                    <artifactId>jruby-complete</artifactId>
                    <version>9.1.12.0</version>
                </dependency>
                <dependency>
                    <groupId>org.asciidoctor</groupId>
                    <artifactId>asciidoctorj-pdf</artifactId>
                    <version>1.5.0-alpha.16</version>
                </dependency>
                <dependency>
                    <groupId>org.asciidoctor</groupId>
                    <artifactId>asciidoctorj-diagram</artifactId>
                    <version>1.5.18</version>
                </dependency>
            </dependencies>

Why the extension is not activated?