By observation in a test harness. when a data connector fails the failover connector is invoked.
The failover connector resolves but its attributes are recorded against its name. when (for instance) a SimpleAttribute comes along to look for its input attributes it looks via its dependencies these include the failed connector but not the failover connector and so in that situation the attributes are not available.
I am not sure how widespread this is, or whether it exists in the wild, but I'm having difficulty not believing that it is 100% of the time.
Fortunately fix is realitively easy, but the best way of fixing it involves making a change to AtteributeReolverWorkContext, which appears to be in -api.
I'll check in a supressed test to demonstrate this shortly and then look and see if I can find a way to make this not depend on an -api implementation. Just in case.
I'll not check in the fix until someone who is not me concludes that this is a real bug.