I've noticed in one of the json config files..
"infill_extruder_nr":
{
"label": "Infill Extruder",
"description": "The extruder train used for printing infill. This is used in multi-extrusion.",
"type": "optional_extruder",
"default_value": "-1",
"settable_per_mesh": false, "settable_per_extruder": false,
"settable_per_meshgroup": true,
"settable_globally": true,
"enabled": "extruders_enabled_count > 1"
},
So, this probably explains what I'm seeing.