Features in the Location Tracking last known location layer stop updating and query results may be incorrect.
This is a known issue. There was a problem with an Elastic Search index.
This issue applies to ArcGIS Enterprise 10.7 and 10.7.1. See: BUG-000131691
The workaround is to update the definition of the last known location layer to reset the maxResultWindow property. This updates the index. Once this is performed, the Tracker mobile app is able to query and update the last known location features as expected. Here is the step-by-step procedure: