Notification from support:
This is not a bug, it seems to be behaving as expected. If you turn off the display of output of your (single) extruder, there is nothing left. The "Extruder" checkbox does not make too much sense for a single-extrusion printer, but there would be "Extruder 1", "Extruder 2", ... checkboxes for a multi-extruder printer, so you can check what each individual extruder does.
For single extruder printers, just keep that "Extruder" checkbox checked.
Question is closed.