No blue dot for unread incoming mail
I'm no longer getting blue 'unread' dots beside incoming messages in Apple Mail 11.2 on my 2016 MacBook. The dots are there in my iOS 11.2.6 mail. I don't see any setting to turn this off or on in Mail's Preferences. Under 'Viewing', there is a setting to show unread messages in bold font, but selecting and deselecting it doesn't change the font. I'm sure the blue dot is supposed to work by default, and it was working as it should until recently. Are other people experiencing this? Is there a fix?
MacBook, macOS High Sierra (10.13.3)