Config schema gives an exception on import:
Drupal\Core\Config\Schema\SchemaIncompleteException: Schema errors for core.entity_form_display.commerce_order_item.c4c_loan.default with the following errors: core.entity_form_display.commerce_order_item.c4c_loan.default:content.field_c4c_interest_payout.settings.formatter_settings variable type is NULL but applied schema class is Drupal\Core\Config\Schema\Sequence
The form display is like this:
...
content:
purchased_entity:
type: readonly_field_widget
weight: 0
region: content
settings:
label: above
formatter_type: null
formatter_settings: null
show_description: false
third_party_settings: { }
Fix it.
Postponed: needs info
1.0
Code
Which formatter plugin was this with? I believe the config schema is correct (sequence makes sense to me anyway), so the problem seems more about how the formatter is saving these values.