I have the same issue. The only work around I have been using is to edit the model, open the tool, click Test, then set back to No_Test. The field mapping works fine from there.
This is really annoy. My automation task stops working when it hits the place. I have to manually run this part of the model.
It appears for me that the model didn't refresh information as often as it requires during execution.
I tried to add in a small python script to refresh the workspace, it didn't work.
Similar situation occurs when I detected a feature class existence. If the feature class has been created in previous part of the model, the Exists function never return true.
I hope that Esri could provide effective info on how to refresh the workspace during the model execution.