Solid-State Transformers with Input-Series/Output- Parallel configuration offer a convenient solution for AC/DC power conversion, thanks to their modularity, scalability and flexibility. However, as known, they are affected by a second-order harmonic ripple due to a local single-phase AC/DC conversion in each module. This ripple can be counteracted by installing Active Power Filters, eliminating the need for oversized DC-bus capacitances. However, in an ISOP SST, dynamic interactions may arise from the coupling between different modules. This paper analyzes their effect on the stability of the APFs. It is shown that achieving stability for a single APF individually is not sufficient to guarantee the stability when multiple units are operated at once, but instead stronger requirements are needed. The analysis is supported by experimental validation, and the results can aid to improve the robustness of future SST designs