Question about 4.20.2 cinematic DOF

Hi. I got a question about the cinematic DOF. I have a masked alpha foliage in the front scene, and a character behind.
I put a cinematic camera with small Aperture to create the depth of field. But I found it has some issue with the Bluring.
see the picture attached. First one is with out Dof, Others is with Dof and camera sittings.
it seems got some rendering issue in certain distance .I marked in Red.
How do I make it right, is there any sittings I could try to improve? Plz help.
Thanks!