If you choose to delete old revisions, it deletes the current revision too
There's this CONSTRAINT FK_civicrm_activity_original_id FOREIGN KEY (original_id) REFERENCES civicrm_activity (id) ON DELETE CASCADE,
on the table so when you delete the original it deletes the current revision too.
I must have never run it with that option.
The cascade seems like a dangerous thing. Maybe it should just be set null.