A colored halo like this is often caused by blur at the subject edges rather than direct flash spill. If depth of field is shallow, fur and other fine edges fall slightly out of focus, and those blurred edge pixels can become a mix of the subject’s color and the gelled background color. That makes the outline look like a halo.

Resizing can make it worse, especially with fur, because downsampling blends those fine edge details and can shift color slightly.

What to try:

• Use more depth of field: stop down the aperture so the animal’s edges stay sharper.
• Check focus carefully on the face and overall subject plane.
• Be mindful when resizing/exporting, since that can exaggerate edge blending.
• If your background light is on-camera, moving it off-camera may also help reduce unwanted edge effects.

Since it only happens sometimes, changes in focus accuracy, aperture, subject position, or how the final image is resized are likely why it appears inconsistently.