Was there an update to morale? Suddenly it seems to be formulating correctly.
Nope, no updates.
Can you think of any recent significant changes to your empire or targets?
Nope, only difference is is that my target and myself both have more planets.
For example I was attacking someone with ~20 planets while I had ~20 planets and the cost was like 15 percent.
Now we both have 75 planets exactly and the cost is only 5%.