Whenever you go from Whole to Split mode, the Roland assumes that, since you no longer have the bottom part of the keyboard for piano, it should drop the upper tone an octave, to leave you a more 'playable' range. Sometimes this is for the best, sometimes not....
The only easy workaround, other than practicing getting your finger up to those hard-to-hit octave buttons on the screen (what were Roland thinking, dropping the hardware octave buttons?), is, if you aren't already using all three UPR Parts, use one for another piano patch, transposed for the split.
For most lead sounds, it makes good sense to drop an octave during a Split change (that last octave at the top is often shrill and unusable), but unfortunately, Piano is one of the few where this behavior is sometimes wrong....
I believe a lot of arrangers have this 'feature', but perhaps some make it easier to defeat?
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An arranger is just a tool. What matters is what you build with it..!