Yeah, it is true that Finnish universities do generally require your Master's to be pretty similar than your undergrad - for example, looking at the admission criteria document from my faculty's international admission pages (it's my understanding that other universities have similar policies), it says that

Following receipt of a reasoned application, the admissions board may grant the right to pursue the Master of Science degree without an entrance examination to an applicant who has completed an applicable Bachelor’s degree with good grades. In such cases, the applicant must meet the Finnish, Swedish or English language requirements, and the content and scope of the completed degree must be correspond sufficiently to the Bachelor of Science degree in the same discipline at the Faculty of Science. Admission will be based on the amount, quality and grades of the completed studies as well as on the letter of motivation.

On the other hand, it does also say that

If the content of a successful applicant’s Bachelor’s degree does not correspond sufficiently to the Faculty requirements for the Bachelor of Science degree in the relevant major subject, the applicant may have to complete up to 60 credits of supplementary major subject, minor subject and language studies before beginning to pursue advanced studies for the Master’s degree.

But I don't know how similar the previous degree has to be in order for them to say "okay, but you have to complete extra courses" instead of rejecting the application outright.