I just did a test and found that the on form submit event it's not being triggered when a response is edited.
I'm not sure if the on form submit trigger is working as intended, if the above is due to a bug or to a glitch. To be sure, post an issue to the Google Apps Script Issue Tracker.
As a workaround, instead of using the on form submit event, use another way to run your script, like a time-drive trigger.
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