Hello everyone! The mechanics were changed with the release of the current patch - now you need to place the champions that have not yet reached their max level in the non-leader slots to make sure that they take part in the battle. We do understand that this all may seem rather confusing, thus, we are going to implement the change with one of the closes releases. When the update is out, you will be able to put the champions of the max level in the non-leader slots and the game will not be throwing them out.
We are also working on fixing the bug with the Artifacts acquired in the battles with the Multi-battle mode on. When the fix has been made, the Artifacts will be sold automatically (if the corresponding option has been selected) instead of going to the Inventory. Thank you all for your understanding and cooperation.