Fluorite-structured oxides constitute an important category of oxides with a wide range of high-temperature applications. Following the concept of high entropy, high-entropy fluorite oxides (HEFOs) have showcased intriguing high-temperature application potential. However, unlocking this potential necessitates an assessment of their long-term stability under high-temperature conditions. In this study, we conducted a prolonged heat treatment at 1000 ℃ on typical HEFO, specifically (CeHfZrGdLa)O_x. After 100 h, high-intensity X-ray diffraction (XRD) revealed a transition from a single-phase fluorite to a multi-phase configuration. Further investigation by analytical electron microscoy (AEM) demonstrated that this degradation resulted from facilitated element diffusion and consequent escalating chemical fluctuation at high temperatures, leading to spontaneous segregation and separation of Ce and La elements, forming Ce-rich, La-poor, and La-rich phases. Notably, the La-rich phase spontaneously transformed from a fluorite structure (space group $Fm\overline{3}m$) to a bixbyite structure (space group $Ia\overline{3}$) at elevated temperatures, resulting in the appearance of superstructure reflection in XRD profiles and electron diffraction patterns. Despite the intricate phase decomposition, the energy band gap showed minimal variation, suggesting potential property stability of (CeHfZrGdLa)O_x across a broad range of compositions. These findings offer valuable insights into the future applications of HEFOs.