strange chroma key effect behavior

After applying a chroma key effect to a clip the effect changes the color of the main subject.

As you can see the color of the sleeve changes when applying the default chroma key effect. I realize the color of the sleeve is very close to the color of the background but that does not explain why the effect is altering any color values. I use the chroma key effect often and have never seen this before. No matter what tweaks (of the many available!) I apply to the effect the sleeve color remains changed and stable. There is no effect on the color of any of the other portions of the clip to be keyed. Double weirdly davinci resolve behaves exactly the same! The original footage shot in 4k on an FS7 to an Atomos Shogun. Thoughts?