This mutation is called on manage tab in time management app.
startDate and endDate parameters doesn't work as search in range but as exact search.
startDate - should be equal to start of the week in your workspace (June 12 on example)
endDate - should be equal to end of the week in your workspace (June 18 on example)
Otherwise you'll get empty array as response.
Check what time offset your query has so it called with correct time, for example on screen above time offset is
Your query on playground should use the same offset, most users shoudl be able to use 12:00:00.000Z as a default offset for their requests.
This mutation available only for workspace admins and if user is not admin for this workspace it will return empty array as result