I’ve done some more testing. One thing I thought was wrong: this still happens with v5.34/32 of ROOT - for the smaller datasets I was able to run them in the 32 bit version and they show exactly the same behavior.
So there is something about the data I’m feeding it in the bib15 files that is causing this silent training failure. I still don’t know what it is, of course. But anything you can see in the training. Whatever happens, the BDT produces infinity for all three multi-class training outputs.