If you look closely at the evidence, you will see that this character, and others which contain the element 冘 with an additional mark, all have readings different from the original Hán-Việt. This indicates that they are meant to be treated as native Vietnamese words with their own reading an nuance, not Chinese. We could handle these cases by adding a new reading mark, but we have not proposed that because 1) there are not many and 2) this method is not generally recognized as a separate mark.