I test the soft shadow,but the Object light shadow is error?

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soft shadow1.jpg
soft shadow1.jpg1452×817 179 KB

First , I want to cast the soft shadow. so I change the value of the Source Radius from 0 to 20. after that, I build the lighting,the softe shadow is ok,but the Object light shadow is error, why?

soft shadow2.jpg
soft shadow2.jpg1496×830 214 KB

thanks for the help.