I have an existing private discussion with Person A, Person B, and Person C.
I return to it later and select 'Manage Participants' and get the dialogue shown - note it says 'Add'. (In pic below ignore Person D - I just didn't screengrab earlier).
I type in Person D (actually Trisha Liu ). Note it does not find her profile and show her avatar.
I press Finished.
Note that now the Private Discussion is just with Person D - Persons A, B, C are gone (- fortunately I can remember who they are).
As a fix I took the discussion into edit and added A, B and C again.
As a comparison on my 5.0 GA
Following the same procedure, 'Manage Participants' shows the existing people and resolves the partially typed name. Pressing Finished correctly adds Bridget and retains Andi and Tony.