I normally wouldn't make a bug report regarding this type of spells, but seeing that circle from protection works, invisibility sphere did not. I casted it, only my character became invisible, for a short 30 seconds max.

Circle of invis gives everyone in the area round/level invis when cast.

There was people in the area. I was the only one invisible.

Were they sufficiently close to you at the time of the casting?

They were, but I'll try again when the opportunity presents itself. Like when I'll be logged on later tonight.

Our implementation of Invisibility Sphere lasts for rounds/level, and has a small radius; on the other hand, once cast, you can leave that radius without losing your invisibility.

I think it's working properly. If it doesn't behave as described above, open up a new bug report.