Hi Timo, if I have a label assigned to the Task that adjusts the price to a constant value and I finish the Task with continuing to work the next day, my price will double. The label does not assume that this is the same task and should be for the same price.
I assume with "task" you mean "work unit"?
Work units over midnight will be split into two separate work units and each of those will get tags (in your case each work unit will get the price adjustment tag).
I don't see this as a bug, I would need to work on this feature request to change the behavior