Software specification quality significantly influences overall software quality. Ambiguity in natural language specifications has been identified as a major obstacle to achieving high quality. Formal methods have been proposed as a solution to mitigate this issue, but their adoption in software development remains limited. While previous studies have primarily focused on the challenges of introducing formal methods, few reports exist on the effects of applying formal methods in software development. To tackle this issue, this study analyzes the trends in OSS (open source software) with and without formal methods, based on issues collected from GitHub repositories. We employed BERTopic, a topic modeling technique, to extract topics associated with issues. Subsequently, we labeled the issues and statistically compared topics of issues in OSS with and without formal methods. The results indicate a significant difference in the tendency of issues between OSSs with formal methods and without formal methods, particularly in the frequency of reported errors. These findings suggest the existence of issues specific to OSSs with formal methods and highlight the potential benefits of formal methods in reducing errors during development.