I created a simple animation with an image plane and a particle effect. The particles are an image of a flower petal with transparency mapped onto a plane. I used the Eevee render engine. I wanted to create a depth of field effect so the petals closer to the camera would be blurry and the ones closer to the plane would be more in focus. That part works; however, when the particles pass in front of the image plane the depth of field completely disappears — they look perfectly in focus. You can see in the gif that the particles are blurred until they cross in front of the plane.
I have tried changing the f-stop and the max pixel size of the bokeh. I also tried it with a regular plane as well as changing the material settings. The same thing happens. I also tried switching the focus from being on the image plane object to just a distance. When I render the animation, the same thing occurs. It's not just in the viewport.
Can someone tell me why this is happening and if they have a work around?