As mentioned elsewhere: the initial subscription was for 12 months. Extending that in NL is only allowed on a monthly basis, or a new contract. This exactly because too many companies made it purposely difficult to leave.

It's fairly logical: the customer agreed on 12 months. An additional fixed term requires a new contract/agreement. In practice companies now try to keep customers. Plus need to actually be ok instead of relying on people not overcoming the fake effort that is required to leave.