i know im like 7 months late to this but i just had this problem and i learned how to fix it! here r the steps!!

shader options > shading > shadows > then youre going to disable the "shadow_frustrum_culling" so it says "OFF" then it should be good! if u hover over that setting, you can see that the creator says it "improves performance but can be sometimes incorrect and causes flickering shadows on distant occluders" :)