My understanding of WebViews is that their storage is separate from the storage used with the standalone web browsers. This is particularly so with iOS, e.g. a cookie that is written in an app's WebView can't be read in Safari, and vice versa. Keeping in mind that the user could also open the WebView's page in their standalone web browser (e.g. via the WebView's share button), I think it makes sense to overwrite the MCID (and AID) so that your user journey reporting doesn't break in that scenario.