It's hard to diagnose long-distance, but you could have a case of Miscanthus Blight, which is a fungal rust disease becoming more and more common to Miscanthus sinensis.

Here's a reply I posted to another forum back in January:

There are numerous citations from the internet, and from Rick Darke's book suggesting a standard treatment with copper and/or sulphur based fungicides may control Miscanthus Blight. Banrot is a wide-spectrum fungicide suggested by Darke.

Trim off diseased foliage as soon as it appears.

Take care to keep the plant clean of dead foliage, especially in fall, and clean up all foliage litter aroung the plant.

Use foliar fungicide sprays and soil soaks before the typical onset of the fungus.