Hi,
In your second screenshot it looks like the topology is not finding all of its boundary coastline features. There can be a few reasons for this, but typically it’s related to the feature IDs in some way.
One possibility is you are using Shapefile for the regular features (coastline) but have not saved it using GPlates prior to building your topology as mentioned in this post.
Another possibility is you have multiple features (coastline) with the same feature ID as mentioned in this post. Perhaps due to a manual copy’n’paste of a coastline GPML file.
If it’s neither of those issues then feel free to send me the files that reproduce the problem and I’ll have a look.
PS: It’s also worth being aware that the menu item View > Geometry Visibility > Show Topological Sections is disabled by default as covered in detail in this post. But I don’t think that’s affecting your problem.
Regards,
John