Variable font mark-to-mark positioning error in fonts.google.com previewer

This is a weird one. In fonts with mark-to-mark positioning such as [Kufam](https://fonts.google.com/specimen/Kufam?subset=arabic&vfonly=true&preview.text=نَ&preview.text_type=custom#standard-styles) and [Markazi Text](https://fonts.google.com/specimen/Markazi+Text?subset=arabic&preview.text=نّ&preview.text_type=custom), the preview page is not correctly positioning the marks as the font varies in width/weight - the marks remain in the same position as the weight progresses.

However, the WOFF file is *correct*, and if you edit the specimen text, the problem resolves itself. It's literally just the way that the specimen previewer is rendering the text. This is both with the variable axes preview and the weight progression waterfall.

An example using Kufam and the string نَ does the following. Notice it appears with the dots and the accent overlapping initially, but that when I click on the text, it suddenly renders correctly:


(Notice also that the Italic renders a fallback, but when I click on the regular both resolve themselves. Curious.)