In Affinity D. forums users are angry because Serif hasn't bothered or been able to include anything which resembles Shape Builder, Blending, Envelope distortion, etc...tools that Illustrator users have used longer than I can remember. Now you have met an existing feature which makes angry.
You may have an exactly fitting seam between 2 shapes, but A.D. doesn't Boolean ADD the shapes to a seamless one, no matter the shapes are placed against each other so that the snapping indicator shows that the nodes snap. After applying ADD you have still a gap, only so narrow that the highest possible zoom doesn't show it. An example:
The vertical seam above was made by Division.
Selecting the halves and applying ADD didn't change the appearance. It has a zero width V-groove.
It can be fixed by adding a patch. Here it's much bigger than needed. One should draw as narrow patch as possible.
The patch is drawn by clicking 5 times with the pen.
After selecting all and applying ADD:
The 4 extra corners can be selected with the node tool and deleted:
There are unfortunately 4 new nodes on the wanted edge and straight line segments between them. They cannot be smoothed automatically. It's best to keep the patch as narrow as possible.
Sorry for showing only a workaround instead of a fix. But I would be interested if someone shows one. Some time ago I thought that having in the preferences option "Use precise clipping" ON would be a fix, but sometimes it still leaves the shown groove if the seam is curved. This time precice clipping was ON. Sometimes applying ADD 2nd time has helped, but not always.
EDIT: There's a comment by user pbasdf ; he suggest editing the common edge so that the halves overlap. That, of course, is an useful idea. In this case it would need only inserting one node to the common edge (=double-click with the node tool) and moving it.
Another comment suggests deleting the common edge from both halves and joining the remaining open paths. That works, but in A.D the deletion happens by selecting the endpoints (total 4) of the common edges, and breaking the paths at the endpoints. Then the unwanted pieces must be selected with the normal selection tool and removed. Finally the halves can be selected with the node tool and joined. Quite complex, I would say, because there exists no such thing as select a wanted curve segment and delete it.
One workaround more:
This one helps also in many other problems that one can meet in Affinity D. The next 2 shapes have the same perfectly fitting seam problem; The seam remains if Boolean ADD was applied:
- Copy and paste the shapes to Inkscape.
- Ungroup. The next image is the result after ungrouping (the black stroke is added manuallybecause Inkscape hasn't automatic edge visualization)
- Do the wanted operation. Here Path > Union is applied:
- Copy and paste the shape or shapes to Affinity D. They come back to the midpoint of the artboard:
The usefulness of step 4 is not at all clear.