dumpVCD has some bugs, but importantly I think the current intended functionality is also not as good as it should be. I started a GitHub Discussion to work on improvements, which also discusses the main problem: the fact that the first clock cycle is right at the beginning of the simulation. I propose changing the current function, which means it can only be done in a new major release of Clash. This issue is to track it as a milestone for Clash 1.10. Actual discussion can go in the GitHub Discussion.