Users may report that they are unable to see all expected activities in Lightning Experience related to a record, with the Activity Timeline displaying the following message:
This issue may occur for all users, including admins, when there's a very large number of activities (thousands of Tasks and/or Events) associated to a specific record.
To verify this, an admin may query the Task and Event objects for a count of activities related to the record in question.
As a workaround, users may select filters such as 'Last 30 days' to return a smaller data set in the Activity Timeline.
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