In both Planning Optimization and MRP functionalities, if the formula ID of an approved planned production order within the frozen time fence is set to blank, the date will automatically shift outside the frozen time fence set in the coverage group.

For example, in the case of a manufactured item that has only one formula, when the formula master is updated, the formula ID of the approved planned production order is set to blank to update the formula to the latest version, and then the same formula ID is re-assigned. This causes the date to shift automatically, disrupting the plan. 

The behavior should be consistent when the formula ID is set to blank and when the formula ID is changed.