See email thread starting here:
http://mail.openjdk.java.net/pipermail/javadoc-dev/2018-April/000472.html

Taglet.init takes two parameters to give it context:
1. A DocletEnvironment, for the tool to provide information about the operating environment
2. A Doclet, intended to be the user-provided (or default) doclet

Currently, if the top-level user-perceived doclet is either the standard doclet or a subtype, the doclet paramter for Taglet.init will be the private HtmlDoclet that the standard doclet delegates to.

The call-sites should be fixed to provide the top-level doclet.