Measure items can be removed when:
The Time Range property for the view changes. That change can impact measure items and time hierarchies.
Your administrator changes the role for your user profile such that you no longer have permission to access the underlying measure for the measure item.
The underlying measure was removed from your Stratum.Connector for Viewer environment and is no longer available to any users.
All measure items will be removed from a view if you change the Time Range property. Changing the property to No lets you use time hierarchies in the view. In that case, all measure items (they will have time ranges) will be removed. Changing the property back to Yes lets you use measure items with time ranges in the view. In that case, all measure items (they will not have time ranges) and all time hierarchies will be removed from the view.
Here is a view that has a Time Range property of Yes and measure items with time ranges on rows.
Here is the view after the Time Range property was changed to No. Also shown is the related prompt that displayed to confirm the change. All measure items were removed. Measure items without time ranges and time hierarchies can be added to the view after this change to the Time Range property.
Your role determines which dimension members and measures you can access in views and other parts of Stratum.Viewer. Roles can be changed over time. If your administrator changes your role to remove access to a measure, then you will no longer see that measure or measure items that were based on it in views.
Here is a view run by a user with access to all Actual Sales and Budget measures.
Here's what the user will see when she runs the view after her role is changed to prevent access to all Budget measures. The measure items based on Budget measures are no longer visible.