When "require validation success" is turned off, invalid emails should appear in your merged column. This issue could be happening for a few reasons:
1. The validation columns are hidden by default to keep your table tidy. You may need to unhide these columns to see the full validation process.
2. The waterfall might be continuing to run after finding an email that failed validation, looking for a valid alternative.
3. Your merge formula might need adjustment. If it's assuming there will be a response for every row (which isn't always the case), it could cause errors.
Try opening the Columns Panel to unhide the validation columns and check what's happening behind the scenes. You might also need to review your merge formula to ensure it properly handles cases where validation fails.