Hi Floriaan,
I will add my name to this.
Every addition or subtraction to the project needs a refresh of the symbol key.
It is particularly annoying as it reshuffles the order of the symbols too, I have a specific hierarchy I use & its time consuming to re-order the symbol key on a number of plots.
It could be placed on the paper underlay but I like use different filters for different plot views so this doesn't help.