I think I've heard others report similar things with this yeast. A cooler fermentation temperature should help prevent this. How warm are you fermenting with it? It should also disappear with aging, after say a couple of months. Even a hefeweizen will lose its banana character after about 6-12 months. If the banana is slight it won't take that long. If it comes out strong, you might need to age it a while. I mean... it's a "lager" after all.
Just try to keep the temperature down below say 18 C if you can, and try to be patient. Even a California lager isn't meant to be fermented hotter than 18-19 C.