PS4 - I purchased the lantern booster twice in a row and after the second purchase the count of lanterns displayed in the upper right corner did not update right away like it did after the first purchase. I checked the in-game shop screen that opens by pressing R1 and it still showed only 1 purchase and 6 available lanterns there too. I even exited Underspire and went back in and it did not update.
Then I played one match using a torch and afterwards everything updated and correctly showed 2 purchases and 11 lanterns available. I made a 3rd purchase and the same thing happened, the numbers did not update until after I played another battle.
So it seems like there is a missing trigger for the display to update after shop purchases beyond the first, and only playing a battle forces the numbers to update again.