I just noticed that Atlas and the two Whitewing sisters can actually depart your party and gives you back all the three items that they were carrying after defeating Blake while haven't defeated Grieth yet in Act 3. I was going to the Dragon Shrine to get the Boy's Journal and promote Genny as a Saint using the Mila's Statue, then these three departed that they mentioned that this isn't the way to Grieth's Cinadel. I was scared at first that they like they did leave Celica's party forever, but you can actually recruited them back at the Zofian Harbor and Mountain Village that they said something like "You changed your mind?". I did have Atlas promoted as a Mercenary and all of them still have the same stats when they leveled up in my playthrough before departing Celica's party.
Also, did anyone tested it on Jesse and see if he does depart your party or not if you didn't defeated Grieth and heading towards the Temple of Mila just like the three that I mentioned? I don't actually recall if the three can actually depart Celica's party to the road straight to Temple of Mila and haven't defeated Grieth yet in the original game or this was added in Fire a Emblem Echoes: Shadows of Valentia?