Well that's good to know that you are able to replicate and that it's a WSL problem. It is still strange that I previously had it working in WSL on my old computer. Perhaps some upgrade broke it.
WSL isn't my only option for jsreport, but it just made since, since the the rest of my dev env was in linux on WSL.
I have moved to use the docker image and it is working fine.
Thank you for your help.