When I checked yesterday, the guest was already verified (in fact, the guest is also an Airbnb host). The stay will happen in some weeks, but it was booked weeks ago, just like a similar booking around the same time. So I suppose it's not related to payment. The dotted line has now disappeared, and I don't see any changes in the booking. Perhaps an undocumented feature.
Mmm, mysterious. I have no idea really. I don't remember ever noticing it on my calendar, but I could easily have missed it as I host long term stays now so don't need to look at the calendar as often.