Hi Sophie, it's probably related to some of these same type of "needle thin" type areas such as here:



Those edges come down to a fair amount of area where the edges on opposite side are very very close to one another:



That type of structure where 2 different edges from the same boundary are so close to overlapping can make some mechanisms consider the trim boundary to have self-intersections, and a trim boundary that intersects itself basically messes up the sense of inside and outside of the boundary, you can get stuff that sort of appears to "leak out" from the boundary since it doesn't know which side is supposed to be the active side anymore.

However in this particular case the confusion seems fairly limited to the display mesher, that's used for preparing triangles for the shaded viewport screen display. If you go to do an export to a polygon mesh format a different export mesher will be used and it's somewhat less sensitive to certain things at a cost of taking somewhat more time to calculate stuff.

If you do a file export like to .obj format for example you'll get what looks like an ok result like this:



So it's possible if you just ignore the messy on screen display and do the file export you may be able to get a good result.

Other than that those needly thin type structures can just tend to cause various problems.

- Michael