This is a know issue with Magento 2.2.7 and 2.3.0 where the "Use Website" and "Use Default" configuration options on store/website scope view are not working.
When you check these options and save the configuration, they will be unchecked and the configuration options are not restored to the default scope values.
This problem only occurs with the configuration for Buckaroo payment methods (e.g. iDEAL or Bank Transfer), and not for general and account configuration (e.g. Merchant Key or Certificate File).
By default the "Use Website" and "Use Default" configuration options are checked, and are properly using the default scope values. The problem occurs only after unchecking and saving these options. As such, we recommend to not uncheck these options and use the default scope.
If you did or have to uncheck these options (e.g. because your store views use a different configuration), you can still manually update the configuration options themselves. Updating the configuration options themselves still works, only the "Use Website" and "Use Default" options are not. As such you will have to manually update the configuration options to the global config values.