You probably name it a feature / basic functionality, but it is that annoying, that I suggest it to be treated as a bug.
When you change costume of a hero, it is changed in all teams it is part of. There are 2 main problems raising:
- You have to be very careful on changing the hero. If it takes part in any of your important team (defense, upcoming war, etc). You have to keep it in mind and change it back, after use. If you forget it (most probably), you might end very surprised.
- If it is part of ongoing war/tournament team, you cant change it at all. Thus if you build a collection of different costumes for a single hero, you can forget about using them for a 4-5 days a week, if the hero is in war or tournament.
From my point of view, it kills the basic idea. Take a costume, that suites best your current need. We will end up just collecting them for bonuses.