User blog comment:Kirkburn/Technical Update: July 1, 2015/@comment-3388044-20150704060938/@comment-452-20150704155944

I assume that that behaviour was when there was the single "preview" button, and that it stopped remembering your choice when they split it in to 2 buttons.

The buttons are labelled "Mobile" and "Desktop", so they skip directly to the "Mobile" and "Desktop" views.