I have had this happen twice now.
The first time was when I tried to do the first Changewing quest. The second time was when I tried to do the 3* blue scale quest.
Right after I tapped Start Battle the game gave a weird error, and instead of taking me to the quest battle I tried to enter, it took me to an arena battle.
All of the dragons were in the upper left position on the screen.
I won both battles, but the game thought I lost them even though all but one dragon survived the first one, and my whole team survived the second. I lost trophies in the arena as well as my streak.
I have since been able to play both quests as normal.
This is a pretty irritating bug given that there’s no way as far as I can tell to predict it or stop it from happening, and there doesn’t appear to be a way to actually win the battle and thus keep from losing trophies and a streak for no legitimate reason.