There is a bug in the new KNIME 3.2 Tree Ensemble Predictor. The "change prediction column option" is not applied properly. It seems to loose it's configuration. Sometimes it works, sometimes not. For example in a cross-validation loop if you reset everything, then reconfigure the nodeit will be correct for the first iteration but subsequent one will use the default column name.
thank You for reporting this problem.
We are working on a solution, in the meanwhile You will have to rely on the Column Rename node to rename the prediction column. Sorry for the inconvenience.