I reproduced it also wih AsciidocFX 1.7.1 on MacOS.
Also, adding `{asciidoctor-version}` to the document allows to see the version of the converter used, here it shows 2.0.8 which is not the latest. But could be somnething else.
Feel free to report it to the repo
https://github.com/asciidocfx/AsciidocFX.