The activity log is not recording the Open Email activity for an email send.
Issue Resolution Email opens are linked to an individual asset. If a record is sent the same email more than once, only the first open activity will be recorded. If you are not seeing opens record for a record, search the activity history to confirm whether or not the asset has been opened previously.
Also, Text-Only emails lack the tracking pixel used to determine whether an email is opened, so Text-Only emails will never show opens in the activity log. In Email Performance Reports, Opens will be backfilled by the report's internal logic if the customer clicks a link in the Text-Only email. Otherwise, opens will not be recorded.