What you were expecting to happen, and what actually happened:
I had a team in slot A selected as pvp defense. Using the “reorder teams” interface I moved a team from lower in the list than A to higher than both A and B. Team B above A became my active pvp defense instead.
How often does this happen? When did it begin happening?
REPEATABLE
And since I know it will come up, “f” the haters stands for… fire ball. It used to just be 4 fire balls. But lately I change it each week to the mythic and color needed for the campaign tasks…
Don’t let the haters get you down. F… ire ball them instead.
So I realized a subtlety that I didn’t know was important last night. I wasn’t using the Teams menu from the main screen (I tried that just now and it didn’t change defense) but rather the team selector menu from a pvp fight. Video is here: Team reordering from pvp fight screen changes selected defense team - YouTube
Edit to add: I just tried the same procedure from the explore fight screen and it didn’t change anything, so it’s particular to the pvp-mode team selection interface.