ServletContainerInitializerDeploymentProcessor.loadSci() has code to read the service file, rather than using the JDK-provided mechanisms, and it does not handle comments. The API documentation at https://docs.oracle.com/javase/7/docs/api/java/util/ServiceLoader.html says that the # symbols and all following characters on the line should be ignored. It also notes the file must be UTF-8 encoded, and ServletContainerInitializerDeploymentProcessor does not appear to force that either. Log4j 2.1 triggers this bug (https://issues.apache.org/jira/browse/LOG4J2-890). Although WFLY-4089 is about the Undertow subsystem, that DUP is basically the same between it and the old web subsystem, so the same change should apply apart from the package/path name change.
The PR is not merged, so it is still NEW.
Verified in EAP 6.4.0.DR11