@Laveure it's a treaty between Indonesia and Singapore https://www.straitstimes.com/asia/se-asia/singapore-indonesia-submit-final-sea-border-treaty-to-un-as-they-celebrate-50th
https://treaties.un.org/Pages/showDetails.aspx?objid=08000002804cce54

In this case, I also fix some neighboring country. You can access it here.
https://indonesia.go.id/peta-indonesia/22