Do you have the same issue, when you use JUP-8 in standalone mode?
Are you having issues with all presets - or certain kind of presets, or certain individual presets?
Is it with Dual and Split presets only? In my exsperience 2 sound presets seems to be able to cause some issues in some presets.
Also is it none factory presets that's old and converted from VC4 or older jupiter versions?
Is it only with sounds that have long release times and so on?
Do you have a way to reproduce the issue? I can only test, if it's not about Reaper, as i don't have Reaper.
I think the Dual/ Split function needs to be improoved.
It's also too hard to tell (or impossible to tell) which sounds are used in a 2 sound preset. It's confusing. For some reason it's even possible to select a 2 sound preset for both upper and lower section in another 2 sound preset. This i think can create issues - including hanging notes.