Whenever a user modifies a Time Forecasting Model to add a metric to the model, the scenarios are deleted because those are no longer valid.
The technical reason behind this behavior is that changing the metrics is changing the definition of the model and is affecting especially the thresholds. For example if the user adds a new metric and the threshold is manually specified, none of the scenarios could work because the threshold is not set and the model would break.
There could be combinations for which an user input is not required, for example when the thresholds are read from the global thresholds table so with some development it could be possible to recognize the situation and avoid the scenario deletion in those cases.
With this in mind fix will require probably more advanced approach to see if the changes made require a new model creation or is safe to add to the existing model.