I think Monsen is on to something. I increased the line width to something non 0 and it get's better. It's still not wysiwyg so I'll play around a bit more with line width and the effect strength.

Thanks for the hint!

Same with me. My main uses for CC3+ are: (Battle)maps for my RPG group and Boardgame prototypes

The later one usually does not have a deadline, while the RPG maps have to be done for the upcoming session. So my boardgame projects are prone to be forgotten for weeks/months when the upcoming RPG session calls for a new battle map...

Sounds about right

Yes exactly: the river in the symbol has a (rather thick) black outline. My Idea was to try and match this outline as close as possible on your other rivers, so it would look like the river from the symbol is part of your own river. at least on first glance, I don't think one would ever get an exact match with this...