Hi,
I encounter the same issue. The first image shows the vector view and the second one the pixel view.
At first I used "Insert inside the selection" to place some shadows onto my objects, but this edge was visible. Now I am overlaying those layers over the other layers and this edge still appears. If two areas with the same color meet, I just overlap them a little, so that the edge is hidden.
I am using Windows 10 Pro, version 10.0.18362 Build 18362, and the latest version of Affinity Designer 1.7.1.404 with precise clipping turned on.
Thanks,
Max