It would seem there is an issue with the borders of Belgium. At almost all zoom levels the France–Belgium and Belgium–Netherlands borders are missing, as are the borders of the Belgian internal divisions and the borders the French region Hauts-de-France and the Dutch Province of Zeeland share with Belgium (for whatever reason the Grand Est-Belgium border and the North Brabant-Belgium border are visible). I would guess that there is some sort of LoD-like issue at play. If you need an example of this the missing national borders can be clearly seen on maps of the German states on Wikipedia (e.g. Brandenburg), which are at zoom "5". As far as I can tell the borders only show up at zoom levels "7" and "9". This issue doesn't seem to be present on openstreetmap.org.
Hopefully this is an appropriate place to report this. (I suspect it should probably be reported on the Phabricator but doing so seems non-trivial.)