You've understood perfectly.
Not an error, I checked the original recording.
I've found the answer in the meantime, but unfortunately, it's not supported by GuitarPro or Powertab.
The solution would be to insert a 1/12 time measure for that one note.
Theoretically, for example: 1/3 is between a 1/2 and a 1/4, so it's a third of the length of a whole note, which equals one triplet quarter note.
Same reasoning for a 1/12, which lies between 1/8 and 1/16.
I hope my midi editing program supports this, or I'll have to compress them to 16th notes after all.