We recently ran into an issue with a client on 8.4 who was doing a back-end SQL data import into the Activity table. The were unable to see any of the Activities from the main Activity Manager page in the web client. The Activities could be seen under the Activity tabs under the Account, Contact, Lead, etc. pages, just not from the main Activity Manager list view.
It turns out that the Activity Manager page expects certain fields to be populated and if they are not then the activities will not show in the list view (even though the count of the activities do show and the filters represent this also).
The fields that are required in the ACTIVITY table are: