Hello again. After a bit more rummaging through. We realized that Mizuchi does not have an accessibility page (shift+a) or the clipboard feature as it did not exist yet in the engine when Mizuchi was created back in 2017. Self-voicing always existed hence it should still work for Mizuchi, but I was unable to get Shift+c to work. Clipboard voicing works in our newest game project, so we know it has something to do with the older version lacking that feature.
I sincerely apologize as this is not something I can easily fix at this time...