This was always set to log in my testings. Now with ACES 1.3 if I apply the reference gamut compression in project settings the issue seems to disappear with image scaling set to Smoother. A few other scaling algorithms still give issues however but much less severe than without the compression.
It could be that gamut compression project wide happens before any image scaling which would explain the differences between that and the DCTL version perhaps.
So we know it’s some negative values issue that happens with scaling whilst combining X/A with Magic Motion. I don’t know how Magic Motion works but it has to be it’s math in conjunction with conversion in and out of ACES. It’s interesting that DaVinci Managed with something like DaVinci Wide Gamut/Intermediate doesn’t suffer from this and even the slider just works as expected then.
If you want me to test anything I’m happy to help but trying to understand/solve it is a bit outside of my knowledge at this point