New AP rejected (non-guess): favored index only range scan

Has anyone ever run across this message in a 10053 trace output?  The access path that was rejected was the one with the lowest cost.  If I force its use by using an index hint, the performance is drastically better.  Why is the optimizer rejecting the index?

I am on 11.1.0.7.  I tried using all rows and first rows optimizer goal with no change in behavior.  The only way I am able to influence the optimizer to use the index (other than the index hint) is by changing the optimizer_index_cost_adj parameter.  I did a Google search and this Russian article is the only result I found: