COM external server: when call one interface ,cannot run in only one thread

Question

I am using a COM external executable that employs Single-Threaded Apartment (STA). However, when calling it from another program, the executing thread is not consistently fixed to the same thread. Upon inspecting all the threads, both the original thread and the suddenly changing thread IDs are present. My setup involves a loop, continuously invoking an interface of this external COM server.Is there an issue with my implementation, or is there an external problem?