I have also experienced transient difficulties when I first switch from Swiftkey to another keyboard. You might have to select the KB you want a couple of times before it "sticks". You can also disable any KBs you are not using: Set tings-apps¬ifications-app info (show system selected from menu in upper right corner). You can disable or uninstall Japanese keyboard layout (and any other unused keyboard layout) if you don't need it. Reboot.
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