I confirm that everything works as expected (i.e., labelmaps are exported with correct indices) when you replace underscore (_) with space. I tested it 5.6.2. If 5.2.2 doesn’t export it correctly for you (with the replacement), you can install 5.6.2 and export in there.
I am not sure how this will fix the conversion to the labelmap though. Labelmaps are what goes into the ML training, and they need to have indices. So we will always need some sort of lookup between segment labels and labelmap indices, regardless if the segment labels comes from a standardized terminology. Or am I overlooking something?