Enterprise Architect allows to draw Parts/Properties of Classifiers as SubLifelines in a Sequence Diagram. This is not allowed in UML.
The option ea2md.graphics.mergeSubLifelines
in the publisher.properties controls merging of those SubLifelines in their main Lifeline.
Set to following option in publisher.properties file:
# If True, SubLifelines will be merged to the main Lifeline. Available values: True, False - default: True
ea2md.graphics.mergeSubLifelines=True