The Optimizer has logic in method ScanOptimizer::useSimpleFileScanOptimizer that tries to determine scenarios where a single subset access will clearly be optimal, and in those scenarios does not consider MDAM. An example would be a query whose only key predicates are equality predicates on a leading subset of the key columns.
However, the logic in ScanOptimizer::useSimpleFileScanOptimizer sometimes rules out considering MDAM when it would be beneficial to do so. An example is a query with equality predicates on the first two columns of the key, no predicates on the third, and equality on the fourth. When the UEC of the third column is small, MDAM is likely to be the more efficient plan and should be considered.