When you delete an activity of any type (task, email, event) from the Activity Timeline, you will not remain on the current parent record home page as expected. Instead you are navigated back to the Object Home or Application Home page depending on how you originally navigated to the current record. This is currently a known Salesforce issue.
Please follow this link and click this issue affects me:
Why am I navigated away from the record I'm working on when I delete a Task, Email, or Event?
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