black regions in cloth sim


I got the latest free content from Epic game :
City of Brass: Environments

but for me, all flags clothes show some black parts ( moving ) and I don’t know if it is because normal or something else .
please look at the picture.
I switch to ray trace but no change.

I turned off the directional light cast shadow and those black parts are gone.
but I don’t know why It is happening!
the static mesh editor shows fine!