The reason this is happening is that when you “trim” a surface, the rest of it isn’t really gone exactly, it’s just disappeared. Trimmed edges are different from untrimmed edges in Rhino in this way - you could hypothetically run the “untrim” command on any one of them to get it back. If you bake something like this and then turn on the points, you’ll see all of the control points even for the areas of the surface that aren’t visible.
In your particular case, you should bake out the hexagon, use the ShrinkTrimmedSrf command on it (which, to the best of my knowledge, will just attempt to refit the parameter space. See Rhino Level 2 Training manual for more details). Then reimport the surface. Since your surface has more than 4 edges this might still result Isotrims off of the edge of the hexagon. In that case you will have to manually create a single (untrimmed) surface in Rhino that approximates your shape and desired UV arrangement (since Isotrim is following the UVs when it cuts. You can check what the general UV space looks like by turning on control points). I’d recommend Loft, but pick your favorite surfacing command.