Fizz W damage procs only once?

Today I was farming with Fizz on custom mode and noticed a strange thing for the first time. When I activate W, i only see the higher damage numbers once. The sound of W-empowered basic attacks continues but the numbers showing damage next to a minion or turret go back to the usual. Looks like 65(physical dmg)-65(magical dmg), 65-150(W-empowered magical dmg), 65-65, 65-65, where one would expect the 65-150 to repeat over the 6 seconds of W active duration. I thought that W helps bring down turrets faster, but if it procs only once then it's quite useless. Is this a bug or am I missing something?