The text of Mordenkainen’s Magnificent Mansion (mmm) states:
You conjure an extradimensional dwelling in range that lasts for the Duration.
Plane Shift states:
You and up to eight willing creatures who link hands in a circle are transported to a different plane of existence.
Can a party plane shift from the mansion to the same plane they where in when they created the mansion? i.e. they create the mansion in the material plane, go into the mansion and then plane shift to a new location in the material plane?