New experience today when booking a B fare via the app (first time I’ve outright booked a B fare in a long time, but it was one of the new differentially priced ones to sell E+ to non-Premier members. Anyway, PN was available on both legs, and so I went ahead with the booking. I selected seats in Y mostly out of habit. After I confirmed payment, I got a message that my seats couldn’t be assigned; when I went into the itinerary, I’d already been confirmed for PN. I didn’t have to click a button like in the screenshot, and in fact I don’t remember seeing anything about Premier Instant Upgrade at all.