Do you use Windows IME?
You have to edit the registry, lemme find you a link. (I use a non-standard layout too so I had to do the same thing)
I hope you don’t mind reddit!
https://www.reddit.com/r/dvorak/comments/9olvdt/windows_10_japanese_text_input_in_dvorak_keyboard/
instead of KBDDV.DLL you need to type in KBDGR.DLL
DO NOT adjust anything else on that screen