the syntax of substr is SUBSTR(<fieldname>,start_pos, length).
since you are providing length = EXP_LengthNotes (which is length of whole string anyways), you are selecting 2 characters not 1 character.
can you try changing your substr condition for expression field EXP_NotesTrimmed to something like this
IIF(SUBSTR(mmc_PayrollNotesML, EXP_LengthNotes, 1 )=CHR(13), SUBSTR(mmc_PayrollNotesML,0,EXP_LengthNotes-1), mmc_PayrollNotesML)
you might need to tweak a bit for carriage return and line feed.