It sounds like you got a dud.

M. sinensis var. purpurescence, AKA M. oligostachyus 'Purpurascens' is usually called Flame Grass or Purple Maiden Grass. Its foliage turns a variety of shades of red/orange in the fall and contrasts with light beige to white flowers. It's a very attractive and compact Miscanthus.

I see that you are in zone 5. I too am in zone 5 and my M. purpurascens rarely gives me any gorgeous colour (this year is an exception).

It has to do with the climate. Here we go from warm summer weather to a frost at the snap of a finger. The grass does not have a chance to change colour because it seems to need a period of cooling weather or else just a longer growing season. If you live near a body of water, that will delay frost.

Frost turns the leaves a tan colour, so if yours goes from green to tan, I expect that is what happens to you too.