I have noticed this a few times since the last patch, in different locations fighting different mobs.
I use the classic client, often running multiple clients. What I have noticed is that in some instances of the client, the mob health bar disappears when the mob invisibles rather than just being greyed out.
First time I noticed it:
Lady M - Client 1 with a disco tamer had no health bar issues, client 2 with a pure mage the bar disappeared each time she invised.
Second time I noticed it:
Medussa - Client 1 with a sampire had no health bar issues, client 2 was the disco tamer mentioned above the health bar disappeared each time she invised.
Third time I noticed it:
Lady M - Client 1 with the disco tamer had no health bar issues, Client 2 with an archer had no health bar issues, client 3 with the pure mage the health bar disappeared each time she invised.
I have not experienced this while playing a single client, though I am also not sure I routinely fight mobs who utilize the invis spell regularly with a single client.
I use the classic client, often running multiple clients. What I have noticed is that in some instances of the client, the mob health bar disappears when the mob invisibles rather than just being greyed out.
First time I noticed it:
Lady M - Client 1 with a disco tamer had no health bar issues, client 2 with a pure mage the bar disappeared each time she invised.
Second time I noticed it:
Medussa - Client 1 with a sampire had no health bar issues, client 2 was the disco tamer mentioned above the health bar disappeared each time she invised.
Third time I noticed it:
Lady M - Client 1 with the disco tamer had no health bar issues, Client 2 with an archer had no health bar issues, client 3 with the pure mage the health bar disappeared each time she invised.
I have not experienced this while playing a single client, though I am also not sure I routinely fight mobs who utilize the invis spell regularly with a single client.