It’s most likely muscle necrosis. Not much is known about the reason this happens. But in my experience, I’ve had this happen to juveniles Amano shrimp as compared to adult big sized Amano.
Also, it’s contagious. If other shrimp eat this dead one, they’ll be affected as well
Update, I removed the two sick amanos and put them in a quarantined tank. I lost one but the one in the video bounced back within an hour. I added a large airstone in my 35g and the other 6 amanos look great. Can’t do a water change yet because they’re working on a sewer line in my street.
Cerelin 
It’s most likely muscle necrosis. Not much is known about the reason this happens. But in my experience, I’ve had this happen to juveniles Amano shrimp as compared to adult big sized Amano.
Also, it’s contagious. If other shrimp eat this dead one, they’ll be affected as well
Update, I removed the two sick amanos and put them in a quarantined tank. I lost one but the one in the video bounced back within an hour. I added a large airstone in my 35g and the other 6 amanos look great. Can’t do a water change yet because they’re working on a sewer line in my street.
How new are those plants? This looks like a pesticide issue