I'm not sure it's technically a bug, it is likely to be a "feature" or inexplicable limitation carried over from Tyros's, but nevertheless you are correct in challenging Yamaha with it because there seems to be no justification for it especially on Genos which allegedly is supposed to be more than just a Tyros update.
Here is an extract from the Multi Pad section of the Genos Ref. Manual:
"Multi Pad—Realtime Recording via MIDI
Before starting operation, note the following points:
• Since only the Right 1 part performance will be recorded as a Multi Pad phrase, you should select the desired Voicefor the Right 1 part beforehand.
• The Super Articulation Voices and the Organ Flutes Voices cannot be used for Multi Pad Recording. If one of these
Voices is set for the Right 1 part, it will be replaced with the Grand Piano Voice when recording."
I believe what you can do is to make a midi using the voice you want, and use Michael Bedesem's Pad Maker Midi to convert it to a Pad. It is a long time since I used it (with Tyros4 originally to use SA2 Clarinet) but it may be worth checking out to see if it continues to work with Genos. (So far I have been exploring other aspects of Genos rather than MPads.)
John