The tuning type is group geometric and it only allows me to change the notes name for note 0 and above, but doesn't allow me to change the name of a note for any note below note 0.
For group-geometric or group tunings, the note names are repeated (just like the relative tuning of the notes) for each group. You can edit the names from 0 to group size, and they will be reflected also for the lower notes.
This is now fixed for the next official 1.29 bugfix release so that you can also edit if the current note is not between 0 and group size.
You can find current test builds in a couple of hours at https://builds.openmpt.org/ . The bug is fixed as of r13977/r13978.
Thank you