I set the component to Build System because this only happens with Bootable JAR and - as jdenise@redhat.com noticed - it's related with Galleon.

This is about some tests in the RESTEasy TS which has been adapted to execute against bootable JARs through a specific profile [1].
The tests are running successfully with WildFly 20.0.1.Final (the ejb layer needs to be removed for this experiment to be run [2]) but fail against 21.0.0.Beta1, so it is a regression.
Also, the tests are passing when the TS is executed against a distributed WildFly version, and only fail when the server is packaged via the wildfly-jar-maven-plugin.

There are 108 tests failing but htis JIRA focuses on those failing because of the following error:

RESTEASY002005: Failed executing POST /all/a/z: org.jboss.resteasy.core.NoMessageBodyWriterFoundFailure: Could not find MessageBodyWriter for response object of type: org.jboss.resteasy.test.validation.resource.ValidationFoo of media type: application/xml;charset=UTF-8

hence I guess serialization issues when the request accept header is not application/json, e.g. org.jboss.resteasy.test.validation.ValidationTest

Steps to reproduce:
1. clone https://github.com/fabiobrz/Resteasy/tree/bootable-jar-support
2. build RESTEasy:

$ mvn install -Dmaven.test.skip=true -Dmaven.repo.local=/home/my-local-repo

3. run the test against WF 21.0.0.Beta1:

cd testsuite

mvn clean verify -am -pl integration-tests -Dmaven.repo.local=/home/my-local-repo -Dserver.version=21.0.0.Beta1 -Dtest=org.jboss.resteasy.test.validation.ValidationTest 

The test should pass
4. run the test against WF 21.0.0.Beta1 with bootable JAR:

mvn clean verify -am -pl integration-tests -Dmaven.repo.local=/home/my-local-repo -Dserver.version=21.0.0.Beta1 -Dtest=org.jboss.resteasy.test.validation.ValidationTest -Ddisable.microprofile.tests -Dserver.home=foo -Dts.bootable

The test should fail
5. run the test against WF 20.0.1.Final with bootable JAR:

mvn clean verify -am -pl integration-tests -Dmaven.repo.local=/home/my-local-repo -Dserver.version=20.0.1.Final -Dtest=org.jboss.resteasy.test.validation.ValidationTest -Ddisable.microprofile.tests -Dserver.home=foo -Dts.bootable

The test should pass

[1]
https://github.com/resteasy/Resteasy/pull/2526/files#diff-8c7ea03eb619e46657b96e8fe8d4f09eR208

[2]
https://github.com/resteasy/Resteasy/pull/2526/files#diff-8c7ea03eb619e46657b96e8fe8d4f09eR262