What you were expecting to happen, and what actually happened:
I switched my first two troops from the Battle screen, without bothering to click Edit Teams. The switch was accepted for that battle, but at the end of the battle, the team reverted to the previous order. I was expecting the team to stay in the new order.
How often does this happen? When did it begin happening?
I was easily able to reproduce it for the screenshots. First time I noticed this. Interestingly, I tried it in Explore and Casual PvP, and there was no bug: the team retained the change. My guess is the bug will affect other Events, as well as Bounty.