I can link to Yugl's really quick Barbed Crescent test and stumble through some of the reasoning.
I've been drinking to get through some holiday family gatherings to the extent that I'm not up to finding the original testing on this stuff. So, in addition to lacking links, this might be off on a finer point even though it conveys the general idea. I apologize to a bunch of people in advance.
The best understanding anyone has of the wonky additional effects right now is that the wonky equation just uses the ~50 magic accuracy given by the relevant base stat. There's no check for whatever baseline is normally used and I'm not sure if there is even a check for normal +magic accuracy.
Obviously, Barbed Crescent has far more than ~50 magic accuracy now for some reason, but still fairly low magic accuracy. Low, but no longer so low that seems like it must be a bug.