Sorry, a little busy these day
i have make some gif and image to explain this
first, our character has 3 facing bones, each one is for one facing
when one facing is visible, other 2 are set to not visible
(though some parts are outside these bones in image below, just ignore them)
our animator set the side one as default and visible in Setup (the N_pose in image)
when playing animations, our animator set all Mesh Slots to not visible under other 2 facing bones
for now we play an facing down idle animation at track 0
and we play attack animation in track 1, and add an empty animation
as you can see the animation played and when it hit empty animation,
it automatically change facing bones visible because empty animation seems set the visible setting to Setup(side)
this not happened if we don't add empty animation
the above image shows that when animation end, it clean the track
but we need mixing to fade out the override animation for better look
without AddEmptyAnimation, it seems impossible