I did that too, even booked everything through booking. But the landlord is a private individual and the buildings belong to various companies. That's why you only need to rent with at least one Month legal. I think it's also specifically about the peace and quiet of the normal owners, to prevent short-term tenants from reducing the quality of living with parties etc. That's why the legal regulations are posted in all buildings. My landlord also said if you have any problems, please contact us directly Contact us and please go to the administration office in the building. If asked, you should say you rented for a month. So as described, short-term renters are illegal.
Because the security in the buildings with hundreds of apartments doesn't know whether you are a tenant or an owner. Especially not how long you rented for.