Platform, device version and operating system:
What you were expecting to happen, and what actually happened:
I got the campaign task to kill 1 Undine and found it in PvP. When the match started the Undine was replaced by a Pet Gnome. After the match I got the announcement about pet rescue but the campaign task had not been cleared. Later I found another Undine and the task updated as it was supposed to.
Don’t battlecrashers count as killing the troop they replace any more? Or is the issue that it was not killed by my troop?
How often does this happen? When did it begin happening?
First time I have seen it.
Steps to make it happen again
Maybe use Gnome palooza while the task is active and you select a team that is known to have the Undine?