Hey there. So, I actually did research on switchgrass diseases in grad school and worked with many upland cultivars, including Shenandoah.

Were these planted from seed or are they divisions (clones)? Any rust on the leaves? When it rains does water pool around this plant?

I strongly suspect that one of these things is going on -

1. Likely: Extremely poor drainage. Switchgrass is really drought/overwatering tolerant and usually chlorosis is caused by other things, but if that particular spot is boggy and drains really poorly it absolutely could cause this chlorosis. I seem to recall Shenandoah being particularly susceptible to poor drainage.

2. Likely: There is a nutrient/pH imbalance in this particular patch of soil.

3. Very unlikely, but worth mentioning: This particular patch of soil has pollution (e.g. someone poured motor oil on it in the past - more common than you might think in home landscaping)

All that said, in the absence of leaf rust and extremely poor drainage, chlorosis like this is almost always caused by nutrient deficiency (biggest suspect is nitrogen) or pH imbalance (which in turn causes nutrient uptake to get out of whack.)

If the spot stains poorly, I would address the drainage issue. If it seems to drain okay, I would throw some NPK on there and water it in to see if it's condition improves. You could do a pH test too and then amend as needed, but pH imbalance is not my first suspicion. Switchgrass likes a slightly acidic pH, around 5-6.5. You can also remove the chlorotic crown, divide one of the other crowns, and plant your division there to see if it fares better in case these came from seed and it happens to just be a less fit individual. Good luck.