Steps to reproduce:

1. Create a new template (you can use the attached test template)
2. Navigate to Discovery -> Item prototypes
3. Select all prototypes and press Mass update
4. Navigate to preprocessing tab in the modal window
5. Check Original box and press add
6. Try to add one more new step for preprocessing

Result:
Existing preprocessing steps would be overwritten with newly added.

Expected:
Additional steps would be created

Probably there was a plan to create the same behavior like it's done for other tabs of the same modal window where you're able to select Add/Replace/Delete but unfortunately this functionality is missing.