UE5.6 animated splothy, noisey artifacts after static light build (see video)

hello,

first time using UE5 (i have some old experience with UE4) my problem is as shown in this video (showcasing normal walls & glitched walls):

weird fast-moving artifacts on some objects and walls (especially those away from direct light and in the shadows, like the back walls In front of the window and door) it happens after a light build .. i’m using an arch-viz template (blank), i’m using the medium build quality, and tweaked lightmass settings to produce higher quality but the artifacts won’t go .. any help would be appreciated.

with best regards.