Reproduce by creating a new view, enabling aggregation, adding a Watchdog Hostname group field, then add Timestamp field as Min or Max. The Timestamp will show as Unix number even though the setting is for Time Ago or default date time.
Now change the Timestamp field to Group (instead of Min or Max), then notice the formatting changes to Time ago or default date times, which is correct.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Not necessarily a solution, but a workaround can be to use twig to render the resulting unix timestamp value as a date format.
Configure Field > Rewrite Results > Override the output of this field with custom text
Set the Text to:
{{ created|date("Y-m-d H:i:s") }}
Replace created with whatever the field name is in views and use whichever PHP date format you wish.
Also you need to ensure Thousands marker is - None -