Possible bug with Phage item

Hello, in my last game there is a possible bug I saw with phage, not 1000% sure how the item works. This is what happened during 1 fight, wukong started his ult on lee sin, I saw this turned and ran as name I casted a spell on me to give movement speed, I reached the back side of the blue buff on the blue side of the map. wukong who was on the red side of the river, when the jg and river meet closest to mid lane. Wukong hit lee with his ult but didn't gain any kinda of movement speed from the phage then after 2 seconds of him moving while ulting still he all of a sudden got the phage passive and closed the gap, https://gyazo.com/bd29deb0775c10e923fd0013d367b96d The second fight wu jumps on me and ults and phage hits him right away. So idk how phage is suppose to work but I'm sure the buff isn't suppose to hit him after a period of time