Thank you for your response!
Actually I’ve solved the problem by inspecting the code in my library and extracting the diff with that on github. I then found that box_roi_pool was defined as
box_roi_pool = MultiScaleRoIAlign(
                featmap_names=[0, 1, 2, 3],
                output_size=7,
                sampling_ratio=2)

In Faster R-CNN code instead, featmap_names=['0', '1', '2', '3']. I really don’t know why this change was made, but changing it solved my problem.
Hope it could help someone else.