I know you said the task status is active, but is the project status active as well? We've never had this issue. Here's a screen grab of how we have ours set up. We give them the ability to log hours for future dates, we pre-populate timesheets with work that is within 1 week of the timesheet's work range. We pre-populate timesheets with tasks & issues that have Planned Dates in timesheet's date range, & tasks that have Project Dates in timesheet's date range.