A couple of days ago something's changed and now on every mobile phone, on every website with the viewport scale attribute set below 1 (which is most commonly) the Adsense vignette ads became cut and non fullscreen.
The only way to make vignette ads display correctly is to set "initial-scale=1" like this:
<meta name="viewport" content="width=device-width, initial-scale=1">
But it makes impossible to change the website's scale on mobile phones. Found no info on this issue. If anyone has any ideas how to deal with it - please tell. Thank you!