Raid boss bar is empty but still shows mobs remaining : 1

Hi All, 

I am reporting the issue , that for me , after leaving a raid while the raid spawning bar is going up , and joining again later , The raid boss bar was empty but it still said 1 mob was remaining. This was only solved by drinking another ominous bottle. I am aware that this issue aldready existed , but the cause was not mentioned.

Thank you , All