Hey there, @Pietia
It wouldn't be possible to pinpoint the exact cause without knowing more information about the shipping settings and the customer order. For example, if you have these rates setup for a Shipping Zone that covers the United States, it will apply to customers with U.S shipping addresses. However, if the customer entered a country that is not included in this shipping zones shipping rates, it would not apply and it would take on whatever shipping rates you have set up for the shipping zone that the country belongs in.
For example, you may have a "Domestic" shipping zone that is just the United States and then a second shipping zone called "Rest of World" that covers all of the other countries. Be sure that the rates you setup are for a shipping zone that covers the countries you want to receive free shipping on orders over $50.
Let me know if this helps!