This is something of a placeholder to investigate the detailed behaviour.
Starting the IdP under Java OpenJDK 9.0.4 or Java 10 EA 46 gives this:
WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by org.springframework.beans.BeanUtils (file:/tmp/jetty-0.0.0.0-8443-webapp-_idp-any-6012741490519251586.dir/webinf/WEB-INF/lib/spring-beans-4.3.2.RELEASE.jar) to constructor com.sun.org.apache.xerces.internal.util.SecurityManager()
WARNING: Please consider reporting this to the maintainers of org.springframework.beans.BeanUtils
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release
The IdP still functions in this environment, or at least my simplistic deployment still runs.
We need to dig into this to figure out which if any other modules are implicated, and see whether we can get updated components prior to our next and hopefully final 3.x release so that we can avoid things failing further down the line.