This is example WCS Field :

This picture is about strat of duel is seem normal

After a have cut-in effect

As picture you will see a triangle at the center of field and normally it doesnt have.

Another example Blue-Eyes Mausoleum Field :

This picture is about strat of duel

After start duel background go black

PS

I test on Window version and not sure about others field have problem like this too

Thankyou as always