Yes, I just verified it - you didn't model anything wrong, trimming those surfaces by the curves has a bug where it doesn't do a clean cut right near the end where there is tangency.

It looks like the mechanism that projects curves on to surfaces doesn't deal with grazing tangent conditions that well (which tends to be a quite difficult area of calculation). However, there is a proper handling of these things in actual surface-surface intersection rather than curve projection. So as a workaround, I extruded your trim curves out (using both sides option) and used the extruded surface as the cutting object. this then generated a good cut. If this seems to be a recurring problem area with projected curve trimming, I may need to internally switch to this to an extrude and surface-intersection method instead of a projection method.

Anyway, I've attached a model using the surface trimming workaround, it behaves well now.

- Michael