Certified GKH Gardening Expert
Unfortunately, these should not brown fully. They should remain mostly green throughout the year. They should also be cut lightly in spring. Cutting in fall will take away their insulation from the cold.
They may have died and are in need of a replant. This article will help with the care of these in the future: https://www.gardeningknowhow.com/ornamental/foliage/muhly-grass/growing-muhly-grass.htm