As mentioned already, the behavior is WAD.
With that said though, we did add an "Army attached" icon in the army tooltip in the last patch. You can see them in your screenshot, just next to the army names. An army will get the icon when it has other armies attached to it (the big icon, with three chains), and if an army is attached to another (the small chain). Keeping an eye on those should make it easier to see if any additional armies will join the battle.
With that said though, we did add an "Army attached" icon in the army tooltip in the last patch. You can see them in your screenshot, just next to the army names. An army will get the icon when it has other armies attached to it (the big icon, with three chains), and if an army is attached to another (the small chain). Keeping an eye on those should make it easier to see if any additional armies will join the battle.