I have a Montage 7 - so I can duplicate what you see given performances that you believe may fall in the wrong octave. Piano-keyed instruments I suppose would best reference the real instrument they are "emulating". So any movement of where middle C is presented would be a quirk (a fixable one).

In my past Yamaha dealings, I've only noticed that at times the guitars would be shifted where I would have programmed them. I do not find myself using the octave +/- key very often when using preset performances (kind of like voice mode in the previous gen. synths) of the Montage.

The nice thing is that this is one of the more straight-forward things to modify as you have a few octaves of pitch shifting that can be applied to a given part then store as a user performance that better fits your hand-position-to-sounding-note needs.

EDIT:

I didn't have the MIDI mapping (note values) to physical keys memorized so I took a look where they physically mapped and I guess from this seat, I expect middle C to fall where they placed the un-transposed, no-octave-change, C3 key. So I would generally have the opposite reaction when a performance treats C4 as sounding "middle C" - I want to shift parts one octave up (again, feeling that some guitars fit this category - maybe other things).

Also, if I did "sit" further to the right of the keyboard - then my left hand would start to get too far away from all of the dynamic controls that are on the left side of they keyboard (scene changes, knobs, faders, wheels, ribbon, etc). This may get where arm length makes some difference in where you would naturally "end up" - not sure.

Summary statement being I do not notice your problem as you've outlined it.

Hello Jason - maybe just me ... I sit with my nose in line with the centre of the screen, which is the centre of the instrument, and C3 is slightly to the left of that. My right thumb falls on C4 with forearm straight - to reach C3 is an awkward thing to do (for me!). Pretty well all the pianos, for example, are pitched too high on C4 for me, so I go down one octave. It's probably my habitual playing style. Interestingly (perhaps!) both the 73 key Korg M3 and the 61 key Tyros 3 place C4 as middle C - or nearest to it - the Tyros is exactly equi-distant to left (C3) and right (C4) of centre! But pitched right - so it seems to me the Montage is pitched an octave too high. Foibles ...