Hey Metrobot, so I haven't printed from reaper but from sibelius or musescore can't remember. I tried now the notation in reaper and this really seems not good. very little control on how to notate rythms, it kinda tells you how to notate.
Can I advise you another tool?
https://forum.cockos.com/showthread.php?t=248475
I did it to use with musescore/sibelius and can be used with any other software that export MIDI.
I really don't advise spend much time with the notation part of reaper, although I very glad we have it. It is really not very good for some things. I think you should make a bug report adjusting the grid quantizing didn't help here although I inputed the right rhythm at the piano roll.
We should really have more control and independence from notation and piano roll.
If you wanna just input your rhythm right use the piano roll
You can use grid in any number you input there:
like 1/4 is 1 grid per quarter note
1/3 is 1 grid per 1 triplet half note
1/20 is 1 grid per 1 quintuple 16th note
1/24 is 1 grid per 1 triplet 16th note