Each time an user edits/opens an already scheduled email, Certain system has "Schedule an email?" auto selected & email trigger time is set to 1 day before event by default. Hence, when an email is saved without unchecking the "Schedule an email?", the email is automatically triggered.
Certain team is working to remove the auto selection of "Schedule an email?" check box.
Until this issue is fixed, here is the best practice we suggest while cancelling/rescheduling an email.

Note- Do not cancel on the schdule emails tab.

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