Added new lines via OM extension using groovy script got removed upon submission of the sales order

Summary:

Created Extension using Create New method to add new lines to a sales order when a specific item is entered. Trigger used is On Save and system behaves correctly as new items were added upon saving. However, when order is submitted and user clicks OK button in the confirmation box, the added lines disappear. Has anyone encountered the same? Any advice on how to resolve the issue?

Content (please ensure you mask any confidential information):

Version (include the version you are using, if applicable):

Code Snippet (add any code snippets that support your topic, if applicable):