There is a system A and B which should communicate, usually via an API from system to system. To Have no coupling between A and B (A should not have implicit knowlegde of what B needs to process and vice versa) there is a "mediation service" C which is responsible for translating stuff from A to B. So C has implicit knowledge of both systems.
Since C is some kind of extra service i don't know if there should be a strong coupling between A and C and B and C or if the coupling between all involved service should be loosely every time.
Loosely coupled: A wants to communicate with B over C ... C is translating the stuff from A and sends it to the API of B ... B "translates" the stuff again to its own internal required format.
Tightly coupled: C is directly transforming the data in the format that B requires to process. C therefore has direct influence of internal processes of B
Going with a "loosely coupled" approach it makes no sense for me why using a "mediation service" at all, it just raises the complexity since there is now some extra translation layer ... or do i miss something?
So my question: Is it common that services which are not living in the same space are tightly coupled? Or is this usually a conceptual mistake?