This happened again, but not with Gale this time. My PC , a bard/ranger/mage has the " see invisibility " spell active. And Karlach used it from scroll.
I went into the counting house, we defeated all the Bhaalists except the 4 that turn themselves invisible after each attack.
I have see invisibility active from spell on my PC and from scroll on Karlach. Minthara is casting dancing lights, and the remaining bankers are detecting presence, but no invisible Bhaalist shows up. My party can do nothing, but let itself be attacked and HOPE that a reaction will do some damage to the attacker.
When I click on the icon of one of the bhaalists to "examine" it, it shows status "revealed", so the invisibility should have been broken. But he doesn't show on the map anywhere.
So it was a long fight, my party doing nothing and waiting for the 4 to attack and get a reaction blow. Only helping and healing our own fallen in the mean time. Only 1 banker survived.
This will make it impossible to get through to the end IMO. I don't think I could get into the Bhaal temple if this is not working.