Thanks for your post. When testing in a test store using the [GraphQL Order Editing] process and adding a variant to the order, the email template is getting sent out successfully after the orderEditCommit mutation is submitted when notifyCustomer is set to true.
So this may be a store specific issue. One thing to check is that the test customer is receiving emails for other regular actions from the store in case there's some configuration issue with the store's Sender Email address.
For direct assistance please reach out [to Partner Support using the email option] and select the 'App Development & API' topic, and include more details like what specific operations were being completed during the order edit, what store and app were involved, and a specific date/time/timezone in the past 7 days when the issue was encountered.