But (for example) there is no evidence for "yn whir". Only for "in gwir". So what we have is a mistake on the part of one person (Nance) being noticed and corrected by others (Edwards and Williams). And then we see "entrenched" views ranging from "No! I won't change!" to excuses for how the adverbial particle can be (even if it never was) used in a phrase glossed in English as 'truly'.
You say that "people" are not taking the word of an expert, but checking themselves. Well in this case, the word of the experts is demonstrably true. And so instead of the experts who point to actual evidence in the texts, "people" are digging in their heels because they are following a different expert, who happens in this case to be wrong.
