Apparently this is by design:
Continuing the discussion from 1.10.3/1.10.4 Update & Alliance Wars Status (16 Mar):
Which is an interesting way of handling it. It allows low level players to set a defense team if they are interested in war, that their alliance fellows can give advice on, or leave empty if they want to skip war.
Then if they level before the actual start of war, they don’t have to race to the prep screen and hope they can set a defense team before the clock ticks over and locks them out of the current war.