Xavier,
A little hard to explain, it would be easy with video/audio.
I loaded a 19-tone-per-octave Scala tuning.
Since there are 19 notes inside an octave that means that, for instance, 13 midi note numbers, say #60 through #72, which normally make an octave, now are less than an octave. 60 is still C4 let's say, but 72 is not C5 but more like G4-quarter-sharp or so. What the FM3 is doing now is giving 19 notes in a row, perfectly, from C3 up the keyboard to F#4 [midi number 66], then leaves G4 through B4 [midi note numbers 67 through 71] silent--nothing happens. Then when I get to C5 [midi note number 72] it sounds the upper octave of the 19-note chromatic scale I was playing. That note would normally have come with midi note #67 [that is, the key G4], and there would be no silent notes.
I hope that makes sense. What it seems like to me is that the software is applying the principle used for scales with **fewer** than 12 notes per octave, in which case it makes perfect sense to omit notes, in other words to leave silent keys.
If this is still unclear I might try to do a short video.
