As a result of the seizure the ownership of the property can change, but as a result of a rent contract, the property might still be unavailabe under unfavourable conditions for the new owner.
It it's a normal rent contract, it will probably get the same treatment as under normal conditions of changing ownership of a property.
If the rent contract is siged in the same period as the seizure, it's less certain that all the rights which the tenant has agains the new owner under normal conditions of signing a rent contract, will count for full.
The person who's property is under seizure, might sign some unfavourable rent contracts, which would be in the disadvantage of the new owner in case these rent contracts should given the same treatment as under normal conditions.
So there might be special regulation about this matter. You can find out for Belgium on the international law forum.