I’m afraid you will have to explain that a bit more, because we don’t have any Japanese experts here.
Doesn’t that change depend on your choice of keyboard you have? Can you send some instructions for us to reproduce this? You will need to be quite detailed, perhaps with screenshots, so we can follow the steps.
When I tried to take a screenshot video, I realized that the problem is caused by Google’s keyboard. iOS’s default Japanese keyboard doesn’t make any trouble. So you don’t need to take any effort for this issue. I’m sorry to bother you.