Radio button and checkbox(true) are blurry in pdf document using openPDF library in Java

anyone can help? In my java code, I switched from itext to openPDf recently. But radio button and checkbox(true) are turning blurry in pdf document now. I tried to replace the png pictures of radio button and checkbox(true) with new pictures, it still has same issue. Checkbox false looks fine. I think it is because there is only straight line in checkbox false.

Thanks