Generative AI (GenAI) offers new possibilities for supporting creativity; however, its limitations in producing diverse and novel solutions have drawn attention across various domains, including creative writing, ideation, and design. Despite growing interest, practical insights into these limitations remain underexplored. By introducing the lens of design fixation to GenAI-generated outputs, we aim to provide a more systematic understanding of its constraints, which have thus far been studied mostly at a macro level. To address this gap, we conducted an experimental study involving ten designers to examine how GenAI design fixation manifests in text-generation and image-generation models at the level of design features. We also analyzed designers’ recognition and responses to GenAI design fixation. We recommend adopting the GenAI design fixation perspective in creativity-focused design research, as it offers unique insights and practical implications for refining GenAI-based creativity support tools.