If I understand the NOS article correctly it's about transferal of ownership, not about a boundary change.

The municipality of Stramproy owned land in this area before 1830, when this land was in also located in the municipality of Stramproy. After the partition, this area became part of Belgium, but the municipality of Stramproy kept ownership of the land.

Now the municipality of Weert, successor of Stramproy still owns the land, but whats to sell it, because it's outside it's borders.