I was able to reproduce this. It happens with all runtimes. When mixing idle -> attack it tries to mix the FFD. Because idle doesn't have FFD, it mixes with the last position of the vertices which is the FFD vertices from attack being applied on the previous frame and the result is incorrect.
In other words, when the FfdTimeline is mixing, it doesn't know if the current mesh vertices have been set by the animation you are mixing from. Currently it assumes they have. If that assumption is wrong, the mixing is wrong.
I see two options:
1) If idle had an FFD key, it would set the mesh vertices and then attach could mix from that to the attack vertices. It is unintuitive to know you need an FFD key if an animation that otherwise doesn't need it will be mixed with an animation using FFD. It is not good that the resulting mix is incorrect without the FFD key.
2) If FfdTimeline doesn't mix from the previous mesh vertices then there is no issue. This means as soon as you go from idle to attack, 100% of the attack FFD will be applied, even if idle has FFD keys.
Unfortunately, neither of these are very good.