I see foliage turn purple on Monarda without it getting cold. It seems it can be caused by other stresses too. You should keep an eye on it and see if it consistently gets purple color more fully and earlier in the season compared to other specimens. If it appears consistent on that plant, you’ll want to trial clones of it in other spots to make sure it isn’t something about that exact spot causing more stress (and therefore more color) to the plant.