Flowing Strike can cause a Tempest to become Invisible to other players, happened to myself several times so far and confirmed by other players that they coudn’t see me.
Also noticed at times there are other tempest not being Visible to myself in BG’s and Vaults.
Can’t say what 100% cause it since i can’t recreate it all the time , but i highly suspect its the Essence - Fluid Vestments that is the main cause than also Server lag might be a additional cause.
Only thing i can say for sure is that when i use the Flowing Strike ability which should make me Vanish for a moment keeps me Vanish till i am able to use the ability again, the rest of the other skill effects are still visible once activated and can be seen by other players or in case of Enemies will be still effected by em.