Moiré photonic crystals support flatband modes with extreme field localization, offering a powerful platform for high-$Q$ microcavities and photonic devices. In realistic structures, however, finite size and unavoidable fabrication disorder break ideal translational symmetry and can strongly modify radiative loss. Here we investigate the radiative properties of flatband modes in a finite, single-cell three-dimensional photonic crystal, which represents the minimal functional moiré-type element of practical devices. By controlling the disorder level, we systematically investigate how the disorder strength affects the radiative loss and quality factor of moiré flatband modes. We find a pronounced nonmonotonic dependence of the average $Q$ factor on disorder, i.e., weak disorder initially enhances $Q$, while stronger disorder rapidly degrades it. Analysis of radiation channels reveals that slight disorder can manipulate the far-field radiation and suppress the radiative coupling, whereas larger disorder destroys the coherence of moiré interference, enhances multiple scattering, and drives energy into the radiation light cone, which leads to a pronounced increase in radiative loss. These results demonstrate the dual role of disorder in moiré flatband photonics and provide clear design guidelines for disorder-resistant high-$Q$ photonic devices.

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• Anderson localization
• Flat bands
• Photonic crystals
• Disordered systems
• Moire materials