Diffusion can be influenced by light beyond the bounds of the image.

In some cases the default mode (Mirrored) can cause glints similar to a gate flare if a very bright source is near the edges of frame. If this behavior is not desirable the Blend property can be reduced or a different Mode can be selected.

Mode - Vignette

No extrapolation beyond image bounds (black)

Mode - Mirrored

Mirrors the image beyond image bounds

Mode - Repeated

Repeats pixels on the border of the image beyond the image bounds

Blend

Decreases the strength of Mirrored or Repeated pixels.