Observed that SequencedMap : SequencedXXX default methods implementation requirements not correctly matching the behavior :

For example :
“Implementation Requirements:
The implementation in this interface returns a SequencedSet implementation that delegates all operations either to this map or to this map's Map.keySet(), except for its reversed method, which instead returns the result of calling sequencedKeySet on this map's reverse-ordered view.
 
Use case – invoking the “add” on the returned SequencedSet
Expected – invocation of “add” method on instance returned by Map.keySet().
Actual - java.lang.UnsupportedOperationException