Slitherine

I've played all 20 turns with these sitting at max strength, but the objective remained ? at the end.

Does anyone know how this objective is met?

You need to identify what scenario and/or campaign you are playing.

Pretty sure he/she is referring to 1st mission of Panzerkrieg where a secondary objective is to wait for the 2 "veteran" units to recover by the assembly point.

As reported somewhere by Aristocrats (can't be bothered to find where exactly), the objective is not meant to complete and is only there to guide the player that that's how he/she can regain the strength of those 2 units without spending RP. Not completing the objective doesn't have any consequences as it's not meant to be completed, but guide the player to this "feature" of the scenario. There are no other ways to conduct text information to the player accessible during the entire scenario other than in the form of objectives, which I find to be something that can be improved in the game, but not a bug.

But I remember: here