Trying to configure parent link and child links in menu
Your 2nd menu link needs the parent property referencing your first custom_forms_entity_export_csv.export route :
custom_forms_entity_export_csv.export_sales_form:
  title: 'Export Sales'
  description: 'Export Sales to a CSV file.'
  route_name: custom_forms_entity_export_csv.export_sales_form
  parent: custom_forms_entity_export_csv.export