I would try the following:
Type Yui in by tapping the keys individually followed by a space
Tap on the name (it should be underlined in red)
Tap on +Yui this will add it to the dictionary
From here, you should be able to use the word flow keyboard to correctly enter this in. This method works, because that name is not in the dictionary.
Word Flow and text suggestions aren't available with all languages. To see if your current language is supported, see Windows Phone feature and service availability.
If your language is in the list, go to Settings → Keyboard, tap on your language and make sure that the "Suggest text" and "Type as I flick through letters" options are enabled.
For more ...
With Windows mobile 10 I had the same problem (I have returned to 8.1).
In Windows Central I found this:
Additional language keyboards may not be present and may not be able to install on US builds after upgrading to Windows 10.
Workaround: Reopen the Windows Insider app and re-select Insider Fast/Slow again. The download of keyboard packages will then work....
So far as I know, this dictionary is "learning" - if you use "too" more than "to", it will automatically type the most used one. But there is no such an option in phone that allows you to customize this.
After searching for answers for weeks, I finally was able to do traditional Chinese handwriting again on my Lumia 640 with Windows 10 mobile.
I first went to settings, language. And add traditional Chinese language.
Then, I went to setting, speech, add Chinese simplified .
After doing this, I went back to settings, then keyboard. You can then add Chinese ...
Apparently from Gabe's blog, this issue is fixed on the Build 14295 (currently available on Slow Ring)