What is siteLaunchModal visits? What variable is it being collected in?
It appears that this is a very implementation-specific question, so we will want as much detail as possible in order to help troubleshoot. Alternatively, you can check out this KB article: Troubleshoot spikes and drops in data
I'm afraid I still can't tell if there's any issue, since I still don't know what siteLaunchModal visits is, nor do I know what variable its mapped to.
When exactly did you see the drop? Can you trend it over time and pinpoint the day? Can you share the trend line that specifically shows the drop in visits? Do you know if any code changes were made, either in DTM or on your site?