I performed the following experiments:
I added a Details filter to a smart object in PhotoShop. I maximized the small, medium and large detail sliders, making the photo look horrible. I then set the Effects Mask to all black, so that the picture looked as though the Details filter was not applied. I saved the filter. The resulting image did not look as though the Details filter was applied.
I then re-opened the Details filter. The Effects Mask mask was entirely white. Its setting was not saved along with the other settings in the smart filter. The image shown in Details had the maximized slider values.
I then confirmed this by drawing black horizontal and vertical lines in the Effects Mask in Details and saved the filter. In PhotoShop, I saw the expected unfiltered stripes.
I conclude that the Effects Mask inside Details does indeed work when editing a photo as a smart object, but the mask is not remembered if the Details filter is re-opened.
Responses to this question seemed to be confused about what mask I was talking about. I was asking about the Effects Mask inside the Details tool. I was not talking about applying a mask to the filter after the filter was created.