The most detailed source for borders is with the Cadastre. Current border is clearly wrong compared to parcel boundaries here: https://eservices.minfin.fgov.be/ecad-web/#/
Via https://www.geo.be/catalog/details/63b98ade-71dc-11eb-8bf5-3448ed25ad7c?l=nl I added a WMS of those borders and fixed the little area here in changeset 137210839