Maybe clean line is a bad description. The area in question is the light strip down the middle of the cat.
The answer is the same as before: depth of field limits. In addition, the obstruction is likely lighted unevenly, or has different shading, from near to far. (Possibly a piece of weatherstripping?) In addition, the far portion, that nearest the cat, is closer to being in focus than the rest of the obstruction, which is closer to the camera sensor.
I'm assuming that if you take a photo of the cat without the obstruction there is no line present.
WM
You can see the cat through the part of the out of focus area, so it can't be anything solid in front of the cat. You can see where the doorframe ends and that part of the cat is hidden .
You would think but the door frame is a hard edge with nothing translucent or transparent, door is open. It is related to depth of field but have never seen it create a band that you can see the subject before. Originally though it was because the door frame was too close for that lens to focus the frame. But was able to recreate the band with a different lens that can focus the frame at that distance. Even at F16 you can see the band.
View attachment 214914