Unlike the compatibility-fw switchable field the new one saves all fields values, i.e. values of unselected fields.

There is this note in https://documentation.magnolia-cms.com/display/DOCS54/Transforming+field+values#Transformingfieldvalues-SwitchableTransformer: "Note that Magnolia discards the values for unselected options when changes are saved. In the example above, only the value entered for  editorplainText or editorRichtext is stored."

Acceptance critera: field stores only one value of currently selected field.