This is a classpath mismatch between the interactive cluster and the Workflow job cluster.
What I believe happened: the notebook was running on an all-purpose cluster with the Maven libraries attached, but the job was using a separate job cluster that did not have (or had conflicting versions of) spark-xml.
Fix
Add the Maven libraries directly to the Workflow job cluster (or job YAML), not just the interactive cluster:
Ensure there are no duplicate or conflicting spark-xml jars
Confirm the same DBR/Spark version is used for both notebook and job