I came across this score with diagonal dotted lines:
Source: https://www.musicnotes.com/sheetmusic/mtd.asp?ppn=MN0016239
Compare with this more typical voice-plus-instrument score of the same song:
Source: https://www.virtualsheetmusic.com/score/HL-17829.html
The meaning of the notation seems to be "sound this note in both vocals and instrument, but cut the vocal note duration short as needed". It is usable, but why notate it this way? Are dotted diagonal lines legitimate notation, or is this a one-off invention by this score writer?