The "newlyAdded" field present in on-disk Replica Set Configs in 4.6 will not be accepted in any reconfig, force or non-force.
This is the outcome of the discussion in SERVER-47331.
Here is the Technical Design that specifies the behavior around the newlyAdded fields
Ban force reconfig with "newlyAdded" fields
Investigate changes in SERVER-47331: Rethink the transition from force reconfig to safe reconfig