Before we could detach the event tree from the final produced file in order to release storage weight into the final file. I think this was managed through a metadata member inside TRestProcessRunner::fOutputEventStorage.
Now if I have a look to what this metadata member is doing I simply see that is doing nothing.
There is this rest-forum thread where we discussed about this.
https://rest-forum.unizar.es/t/behaviour-of-inputanalysis-inputevent-and-outputevent-in-trestprocessrunner/187/3
Hard for me to track when this feature disappeared.