Blurred DOF area of foreground elements is warping the background.

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jpg_fram

Houdini 12.0.626 on Linux

Camera "Sampling" with "radial" or "box" Bokeh gives a similar result.

Mantra Surface Shader applied to yellow spheres without any
SSS, object reflection, refraction, emmission, transparency, or displacement.

Motion blur disabled. DOF enabled.

All Mantra render engines give a similar result.