Edit string data set to 24 frets, but can't move fret 19 from higher string down one string

I have a score imported from Guitar Pro 7 where the notes were all supposed to be on the 6th string, but on import they were shifted to other strings (presumably the lowest fretboard locations). I tried moving them using Ctrl+Down arrow, and got them all eventually except for the 19th fret on the 5th string, which should be the 24th fret on the 6th string. Seems like it's treating "0" as the first fret and "23" as the 24th fret. Oddly, however, I can enter 24 on the 6th string via note input mode, even though I can't move the existing 5th string, 19th fret note to that position.