Resolved by reverting the upgrade to Java 11

Thanks I'm still rather inexperienced with Maven.

I may investigate moving the API style tests over to use the failSafe plugin and the integration-test phase.

Yes.
Example for raml-module-builder/util submodule, output taken from mvn --debug clean verify:

surefire:
boot(compact) classpath: ... test-classes classes ...

failsafe:
boot(compact) classpath: ... test-classes util-31.1.0-SNAPSHOT.jar ...

maven has these phases (taken from https://maven.apache.org/guides/getting-started/maven-in-five-minutes.html#maven-phases ):

• validate: validate the project is correct and all necessary information is available

• compile: compile the source code of the project

• test: test the compiled source code using a suitable unit testing framework. These tests should not require the code be packaged or deployed

• package: take the compiled code and package it in its distributable format, such as a JAR.

• integration-test: process and deploy the package if necessary into an environment where integration tests can be run

• verify: run any checks to verify the package is valid and meets quality criteria

• install: install the package into the local repository, for use as a dependency in other projects locally

• deploy: done in an integration or release environment, copies the final package to the remote repository for sharing with other developers and projects.

surefire is the test phase and tests the .class files, failsafe is the integration-test and verify phase and tests the .jar file.

I don't really understand the difference between surefire and failsafe. Do they define a special meaning for unit and integration tests? And tests executed by the failsafe plugin execute the packaged jar?

mod-circulation has unit tests only: *Test.java executed by maven-surefire-plugin.
It doesn't have integration test: *IT.java executed by maven-failsafe-plugin.