If you use a Smart Filter’s mask to hide the effects of a Smart Filter, Photoshop will still make selections based on the filtered content – even though it is hidden.
Here is the original photo of an Iceberg. I’ve converted it to a smart object so that I can add the Path Blur filter as a Smart Filter.
Below shows after adding the Path Blur (listed as Blur Gallery on the Layers panel) as a Smart Filter. Notice that the entire layer is blurred.
I drew a linear gradient in the Smart Filter’s mask to reveal the Path Blur in the water, but hid it from the iceberg.
Using the Quick Select tool, I expected Photoshop to easily select the sky, but it selected the iceberg as well (because Photoshop applies the blur to the entire layer – the mask was only hiding the filter).
Hiding the Path Blur (by toggling off the eye icon next to Blur Gallery), enabled the Quick Select tool to easily select the sky.
In the final image below, I added the new sky layer, used the selection to add a mask so that they sky wouldn’t overlap the iceberg, and toggled back on the visibility next to the Blur Gallery to display the Path Blur filter in the water.