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Timelines on the new Dynamics 365 Unified Interface (2019 release) show all activities, not necessarily those directly associated with the record it’s being displayed on.
The request is to allow a display of only directly related activities on a record timeline.

More information regarding this issue:
According to Microsoft support engineer, this behavior will be addressed if more customers will vote for this idea.
The behavior was confirmed to be a system design and considered a feature gap.
it is a result of the possibility of inherited relationship from other records such as user, accounts, or system associated with such events which get posted on the timelines even though not directly connected.
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