I have now played the appropriate part in v1.2 of this mission again. When you leave the bank with both evidence scrolls and place them in the little storage basket on the Mayor's desk, you should get teleported to the Old Quarter (the other section of the city, where you started the mission), beyond the gate. If you are still in the second part of the city with the bank, the Mayor's office and the City watch station after planting the evidence in the Mayor's office, then something's wrong.

I played on easiest difficulty level, and got an optional objective: Bring the painting to a shop (near the museum and the City watch). This, however, is optional, so once you have been in the Mayor's office, leaving the evidence there,, you can't fulfil this optional objective anymore if you have not done so already. But that should not prevent you from getting teleported into the Old Quarter.

What you describe sounds like a bug, indeed. The map highlighting the waterworks is okay since you would have to go there next. But since you never got teleported, you can't go there.