User is trying to allocate a task to an employee, but the task disappears from the list view as soon as it is saved by clicking Done.
This can happen if the date range set in Period in the Allocation & Forecasting > Resource Allocation > Tasks screen excludes the date range of the task. If you did not use any Start Date and End Date for the task, then you should make sure that Period is set to All Dates in order for the allocated task to be visible in the list view, as all other date ranges exclude such tasks. Check CORE Help Center for more on Allocation & Forecasting.