It happens because of the sound (Tone).

Roland, in their wisdom, decide which sounds are more likely to be needed in a different octave when you change from Whole to Split, and unfortunately, it isn't always the ones you want....

The same weird logic applies when calling Tones up.... with no warning on the Tone name, if you are in Split mode, some sounds will come up in different octaves.

This used to be easier in the G1000 days, because there were a couple of physical buttons for octave Up/Down, but now they are on the touchscreen, in a very poor location. It's one of my requests to Roland for the next G-series that they return those buttons to the Keyboard Part section.

Who knows? Maybe they will see the light.......