Root cause: 

The logic of health relation or and should be, health = main || (add1 && add2 && add3 ...).

But we only check unhealthy in source code. As a result, if there is any health check is unhealthy, the status will be set to DOWN.
Solution: 

Rewrite the logic of or and part.

When main is UP and others are DOWN, it should be UP
When main is DOWN and only one additional check is DOWN, it should be DOWN.
When main is DOWN and all additional checks are UP, it should be UP.