Had a warrior join my scouts army through an ingame event, after the warrior joined the army I started a skirmish, upon starting the skirmish the enemy retreated and a preview window popped up and both my army and theirs went invisible, fixed it by reloading the turn.

Seconded. Bug that carried over from the VIctor OpenDev

Other than loading, they can also become visible again if:

- you get into a battle with the invisible armies