Plants can still be dormant in spring, because the weather can still get cold in spring. If the plants stay that way into the summer, then you might have a problem.
Now, there is a known bug that leaves plants stuck in the Harvest state, but makes them unharvestable. This is associated with a variety of things, such as a fairy's Bloom ability, having the Gatherer trait, and other things. The plants usually aren't shown frosted over, though.