My Sage comes with a double spouted portafilter, fed as per the photo by a single hole in the middle.

If I drop some water into the pf it pours out of each spout equally, but when I make a coffee, the pour only comes out of one spout for the first 20 seconds.
Why? I do not usually stir with needles but I just did. I level the bed, tamp evenly yet the coffee I made did not start to come out of both spouts until 22 seconds or so. If the bed was not level or had harder ‘spots’ in it and was saturating and pouring from one side (until the other catches up) then once it enters the single hole you would expect it to come out of both spouts?