Many issues identified:

• if you define your own ftl for the extra, it's simply overridden by the migration
• extra 1 is not created for a page template, extra is in fact not well created for a page

Things has to be reviewed in info.magnolia.migration.task.stk.TransformStkBasedTemplateMigrationTask.transformExtrasToSubAreas(Node)

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If you set on extra another_extrasArea.ftl, after the migration the extras configuration must be like that:

And in the report you must have a message that tell you to review another_extrasArea.ftl: