Can also be clipped highlights and highlight reconstruction (the module, not in filmic) failing. Try this: Magenta highlights vs raw clipping indicator vs filmic white level - #84 by 01McAc
With highlights saturation mix at 0:
+50%:
-50%:
It’s not caused by filmic, but the module makes the problem more visible. Exposure dropped, filmic off:
Contrast EQ sharpening to bring out the sharp transitions:
CA is similar, filmic can make it more evident. It goes away / is much reduced with highlights saturation mix = 0, or filmic in v6 mode and norm = luminance Y or no preservation.