Hi,
reseating the individual fans doesn't change the behavior. I also tried swapping a fan with a green LED with one with a amber LED. This resulted in even weirder behavior since it changed the status of one of the other fans. Swapping them back brought back the old LED pattern.
While pulling the fans the chassis error LED came on as expected - so that seems to work. When all the fans were reseated, the chassis error LED went out again and like before, nothing indicates a problem except the fan LEDs themselves.
So for now I'm going to assume that this is a bug in the component that controls the state of the fan LEDs since I can even see all the fan RPM values from the CLI - they all look perfectly fine.