Could be any number of things-
Could be a different variety- things are mislabeled all the time.
Could be lack of nitrogen- have you tried feeding nitrogen and/or iron? Maybe some chelated minerals?
Could be a pH anomaly in that one tiny spot.
Could be a buried troll.

If I had to guess I would say it was mislabeled and is simply a different variety or a sport.
It is walking and talking like a duck after all :)

to the left.. they are next to grass.. cool and cozy .. well watered grass ...

to the right.. they are shaded.. in the pic..

is this one out there in blistering sun.. retained by the brickwork ... heat retained late into the evening???

if so.. its more stressed than the rest

i had a west facing dark brick patio.. and i swear you could fry on egg on it.. after midnight.. in july and august ... and walk out there barefoot.. half the MI winter ...

can we also rule out any snow shoveling issues???

did it just change color .. as the heat of summer hit????

ken

Discoloration of box often due to root rot. Your problem shows why geometrically symmetric plantings are to be avoided whenever possible - as soon as part of the grouping develops a problem, the entire effect is lost.