WaterBody Reflection issue

My level is UE4.26 waterbody(lake) under the skysphere. I want to substitute the texture of skysphere, disapear the moon, and at the same time, water become still like mirror.
I change the texture, remove the moon and modify the water material’s parameter ‘Default Near Normal Strength’ to 0 in levelbp, and the lake indeed like a mirror. But when camera moving, the lake surface nearby looks weird, it’s blur and still reflect the former sky texture and even the disapeared moon.
Does it’s a mechanism engine cache the former reflection, or some other? How can I get the mirror lake after a change to the content it reflect? Thx!