Roland JX305 arpeggiator wierdness

Hi folks,
I’m trying to understand the arpeggiator of that box – it has a setting for upper, lower and both as a target setting which part to address when using the keyboard in split mode. I think I understood it right, the manual says the same. Nevertheless arpeggiator is only addressing the lower part when in split mode. No matter what this parameter is set to… bug? Feature? Pebcas (problem exists between chair and synth)?

Wierd stuff all that…