I have Synchronized Strike specced with Dark Allies (+2 shadows) as well as Shift with Lasting Presence (leaves a shadow on Shift). Together this creates 5 shadows - 4 at once with SS and 1 a little later when I use Shift. When the first four of these shadows expire, they leave an Umbral Blade in the ground as expected. When the one created by Shift expires, it creates a blade as well but then one of the previous four blades left by a Synchronized Strike shadow also disappears.
This happens when I just wait for the shadows to expire.
If I use a skill like Shadow Cascade, the last shadow to perform the skill will leave a blade and the blade from the first shadow that performed the skill will expire.
If I use Umbral blades/Shurikens (basically a non-cascading skill), all 5 shadows expire at the same time and all of them leave an Umbral blade in the ground.
I have not allocated Cacophony of Steel so this shouldn’t be due to the bladestorm limit.