Certain task assignments have issues updating ETC values.
The update for per-period Assignment values do not save. Any value entered would be not be saved/changed.
Go to the task properties and review the start and finish dates.
If it is outside the range, the field will not be editable.
Go to the task assignment and click on the properties icon.
Review the ETC Detail.
Remove/configure the rows shown to enable the update/save of the entered values values.