I can't help with this specific question either but on a different thread yesterday or the day before, there was a picture of a variegated arabis which I grow as A. ferdinandi-coburgi 'Variegata.' There is another form called 'Old Gold' with yellow variegation instead of white. Why I mention them here is because in their mats, both will occasionally produce some plain green leaves instead of variegated. These are not seedlings as one might expect, but sports. So maybe yours, David, is doing the same thing and sporting.