Issue
A Change Data Value activity gets recorded on a person record but the Updated At field for the record doesn't get changed to reflect it.
Solution
The "Updated At" field in Marketo is a system managed field that gets refreshed any time that there is a data value change to a person field, but not when the field is on the company object instead. To confirm what object the field in question is on, search for it in Field Management and check what folder it is located in. The folder Info (or Lead Info) contains person fields while company fields are contained within the Company Info folder.
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