This is a frequent bug and it’s surprising it has not been fixed yet.
About 3 days ago one of my tasks changed by itself after a battle, I did not touch it and left it for later to complete as it required me to play 3 times Allies in Moscow campaign (also there was daily box reward attached to this task - 2x silver order for something).
I did notice a change when after one battle I’ve got task to “Complete 3 battles” and I already had 1 out of 3 part done, also 2x silver order was gone from that task (I did not claim it before as I was still yet to complete the task).
For the rest of the day every time I was trying to change task for something else I only could do that once despite having 3 or more tries at a time. I’ve never had any problems with this so far (for about 5 months of almost everyday playing), definitely something wrong is going on lately with battle tasks.