This happened on my butterfly last year as well. From observations it seems to be fairly common on variegated plants that they will revert back to the standard form in places. I know comparatively very little of A.palmatum cultivars, so couldn’t say whether there is anything distinguishing from the species plant in this reversion, but I suspect not particularly. Looks like a nice, healthy branch though so I’d probably layer it off and let butterfly (which doesn't seem to be the most robust cultivar) do its thing.