While talking with my wife recently, the friendship level pop-up in the dialogue box shows we have level "13/12":

This seems peculiar, since only 12 hearts out of a maximum of 12 show in the social tab:

Furthermore, on the wiki (here and here), the maximum friendship level is stated to be 12 for a spouse. From the second link (emphasis mine):

Once a villager has been married, they will move into the farmhouse. They will have a maximum of 12 hearts instead of 10 [...]

Is this "13/12" intended? Is there a secret 13th friendship level or something, or is this a bug?