WildFly 12 Wicket 7.10 - Why does only the first thread have access to Application?

Question

Each request attempts to create new instances of Singletons, which fail because they use Application.get() which fails with error:

There is no application attached to current thread default

Same code works in WildFly 8.2 and Wicket 6.20. So two problems:

1. Each request handler thread acts as though it is running in its own JVM not sharing static fields with other threads.
2. Each request handler thread after the initial one is being created without and application attached to it.

Minimal example:

package com.example.web;

public class SiteConstants
{
   public static final String CONTEXT_PATH = WebApp.get().getServletcontext().getContextPath() + "/";
}

Home page uses

SiteConstants.CONTEXT_PATH

First request for the home page works. Second call loads in another thread and fails on

WebApp.get().getServletcontext().getContextPath() + "/";

because WebApp.get() throws

org.apache.wicket.WicketRuntimeException: There is no application attached to current thread default task-1