Could be a Graphic Override, which sets the Fill Type of Fill elements to Solid or transparent?
I checked the default ARCHICAD Template File and several of the default Graphic Overrides will cause this effect. So if you activate a Graphic Override or a saved View in the 3D Document Viewpoint, it can modify how your 2D Fills are displayed.
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2x28" (2560x1440), Windows 10 PRO ENG, Ac20-Ac27
