I'm uncertain if this is the proper behavior for this or not, but when
you perform #keysDo: on a MappedCollection you get the mapped key values
which are different from the response from #keys.

Example:

st> d := Dictionary from: { #aaa -> 1. #bbb -> 2. #ccc -> 4}
Dictionary (
        #aaa->1
        #bbb->2
        #ccc->4
)
st> m := MappedCollection collection: (Array new: 4) map: d
MappedCollection (nil nil nil )
st> m keys
Set (#aaa #bbb #ccc )
st> m keysDo: [:k | k displayNl ]
1
2
4
MappedCollection (nil nil nil )