The graphical user interface (GUI/UI) provides a visual bridge between the application and its end users, and users can use the application through interactive operations. With the development of mobile applications, GUI, which combines aesthetics and interaction design, has become more and more complex, and users are increasingly concerned about the accessibility and availability of applications. However, the complexity of GUI also brings great challenges to its design and implementation. Due to user-defined settings for mobile devices and different device models and screen resolutions, UI display issues frequently occur. For example, due to software or hardware compatibility, when rendering interfaces on different devices, there will always be display issues such as text overlap, component masking, and image loss. They have a negative impact on the availability and accessibility of applications, resulting in poor user experience. Unfortunately, little is known about the causes of UI display issues of mobile applications. In order to cope with this challenge, this study collects 6729 screenshots of applications with UI display issues from Baidu crowdtesting platform and 1016 screenshots of applications provided by issue reports in GitHub and identifies nine types of UI display issues using the theme analysis method. Through the analysis of 1061 UI issue reports from GitHub and the corresponding defective code, the essence and causes of UI display issues are summarized. The research found that (1) 62.1% of the total screenshots in crowdtesting dataset are defective screenshots displayed on the UI; (2) the reason for the UI display issues is that the font scaling setting does not match the adaptive setting of components to a great extent; (3) the layout setting of the interface will lead to display issues; (4) If the hardware acceleration is not turned on, the normal display of the interface will be affected.