In my experience, they tend to shoal tighter in light. As soon as the light goes off at night, they are much more independent and swimming about. It doesn't appear to be a problem, just a behavioral thing.
My rummynoses have been fighters in terms of disease. They don't appear to be affected by NTD, and most have survived an Ich outbreak. I generally treat them as I would any other small tetra.
Their noses are a great indicator of their comfort. Keep in mind, if you switch on the light while they were sleeping, their noses will be pale.