After some discussion, I came to the conclusion that this is a limitation of ARCHICAD, but also a design decision. While the representation of other elements shown in floor plan (for example: Roof) is mostly symbolic, Morph is 3D projected. Unfortunately, SEO's effect is hardly displayed correctly on the floorplan.
At the same time, the Morph is defined to be a monolithic element, therefore if you cut a morph, in 3D, the cover surface will be visible in the cut part. If this cut part is visible in the floor plan, the cover fill will be shown here, due to the nature of Floor Plan Display
(Projected: shows cut part of the element’s 3D model, plus its uncut (downward) part).
In short, this issue is a limitation (SEO effect on floor plan display), and a design decision (Morph is monolithic) at the same time.
I hope this will be understandable! Let me know if there is any further question you would like to ask!
Have a nice day, and stay safe!
Technical Support Engineer