Hello Erik, Thanks for posting and for your explanation. As a community of cartographers, we are always paying a lot of attention to such things as "realism" in fantasy maps, so I think you will understand the concern. Even in a fantasy story, the world has to reflect the realism that makes it believable to people living in our real world.
I'm sure it's too late now, but there are ways to accomplish what you described (about two rivers refusing to share the same mouth) and still maintain the realism of rivers always flowing downhill at every point. Perhaps if you need another such map, you might see if you can find a way to do that. Otherwise, to knowledgeable eyes like the ones here, the map just distracts by its "wrongness".
As a life-long fantasy reader, I'll look forward to seeing your books come out in English!