This could be happening because the email found by that provider failed the validation check. When running email waterfalls, all found emails go through a validation process (using ZeroBounce by default). Even if a provider finds an email, it won't appear in the final merged column if it doesn't pass validation.To understand what's happening behind the scenes, you can unhide the validation columns in your table. This will show you how the validation process is working and help you see why certain emails aren't making it to the final merged column.
Sources:Enrich People (Waterfalls) [Automated Outbound] - Clay UniversityEnrich People (Waterfalls) [Clay 101] - Clay University