Yeah, it's with sprites which are on more than one layout.
I was interested to read this, as I wasn't sure this was the case whenever I've tried to access animation.
So I created a new project from scratch, went to the animation tab and added a few frames to the default animation without a problem.
This is a first, so there may be something in what you say, but there's still a major problem with adding new animations.
In the same brand-new project, I added a new animation, and was able to change the graphic for the first and only frame in this new animation.
But as soon as I right-click to select "add new frame", Construct freezes, and that's the end of that.