If I allocate extruder 1, to the bulk of a project and for argument's sake extruder 4 to a Height range modifier, 
Prusa slicer ignores the height range modifier

If I do not allocate an extruder to the bulk of the project, Prusa slicer automatically applies the Default Extruder, which happens to be extruder 1
then prusa slicer  actually applies the Height Range Modifier, as expected

split Butt Extension

regards Joan

Escalated to Prusa