There does seem to be a bug here. If I have a collection with 5 metered
articles in it and my daily threshold for preview is set to 1 - I can
open the first article, fairly quickly swipe through the others (within
that 15 second window I guess) - and then return to any and all of them
to read and will not be presented with the lock icon. It seems to get
around the preview threshold. It's why I found the behaviour
unpredictable earlier.