Hi grjusk, adidam,
Welcome to Affinity Forums
In this case another option is to add one node to one of the yellow shapes over the shared edge then drag it over the other shape so part of it overlaps the second shape then performing a boolean operation between the two so they become a single shape since both shapes have the same colour.
Another way to disguise the issue is adding a background shape with the same or similar colour (depending the complexity of the images) as the ones in the foreground below that edge.
@adidam
If both shapes are aligned to the pixel grid and you are dealing with a perfectly vertical or horizontal straight edge those gaps shouldn't be visible at 100% zoom nor should appear in the exported images (assuming you don't rescale them).
I know it will take time, but here someone will share his workflow for using Designer and After Effects. If you haven't solved your problem by the time the articles is finally there, you can check it out. That might help:
https://www.schoolofmotion.com/blog/illustrator-vs-affinity
Best regards!
adidam reacted to MEB in Joining objects without free space between them