WFCORE-6221 and WFCORE-6347 were developed concurrently. When WFCORE-6221 (merged into upstream first) was merged into the branch for WFCORE-6347, the SubsystemPersistence factory method for manual parsers/writer was not handled correctly - affecting the ability to correctly locate a writer for a given stability level.
Since this is new code, no subsystems are affected by this, but until this is fixed, subsystems that use manual parsers/writers cannot use org.wildfly.subsystem.SubsystemExtension.