Invis/sahauri's sonorous clouding (bard invis) stacking issue

[Not Sure]

So, I'm not sure where the issue is in this, the way it stacks or the song overwriting invis, but there seems to be an issue. If you attempt to invis while the bard song invis is on, it returns the "you are already invisible" message. If you already have an invisibility spell on when the song begins, the song will not overwrite the spell, so they'll stack. However once the bard song fades, it is as if the spell also faded, you appear with the spell effect still listed in your buffs.