Well technically the expected thing is happening, since all menu entries and shortcuts are named "+/- 12", not "octave +/-1". I'll see what can be done about applying instrument settings to the transpose and find&replace octave options though, but might be somewhat tricky to implement. Note that there is already a "transpose custom" option which you can use to transpose any amount, it's just one more key press for the time being.

Ahh, okay! You're right. Thank you for pointing that out to me.

Okay, the transposition and data entry shortcuts now take instrument tuning group sizes into account. Same applies to transposition in the instrument's note map.
You can grab an updated built to try at http://buildbot.openmpt.org/builds/

I'm sorry if the bug report wasn't good enough, it's my first for any project, and I'm still new to OpenMPT itself.

Awesome: I downloaded it and it worked exactly as it should!
Thank you!