Steps to reproduce:
1. Create new Paragraph "jobsParagraph " within Magnolia Module Samples;
2. Define Custom Dialog "jobsEditDialog" for this paragraph;
3. In the source code of Magnolia Module Samples SNAPSHOT 4.2 add new class JobsEditDialog inherited from ParagraphEditDialog to be able to override the method ParagraphEditDialog.onPreSave. Re-build new Magnolia Module Samples SNAPSHOT 4.2 and install it in Magnolia.
4. Configure this new custom dialog "jobsEditDialog" to use newly created class JobsEditDialog instead of ParagraphEditDialog - set the class datanode JobsEditDialog.

Result:
System ignores class datanode here, It uses modules/adminInterface/dialogs/editParagraph/class or selectParagraph/class instead when creating or editing new paragraph.
However system uses all other information from samples/modules/dialogs/jobsEditDialog when constructs the custom dialog.

See screenshots.