I have this problem on two computers now. I had the Japanese language pack installed, and used the IME keyboard. I used to be able to use CTRL-CAPSLOCK to switch to Hiragana after changing the language to Japanese. I could also right click the A icon in the system tray and pick Hiragana from the menu. This allowed me to type romaji which were changes to the appropriate hirgana/kanji. But after installing the April 2018 update 1803, both methods don't change from half-width alpha-numeric.
There were computers running Win10 1803 with the Japanese language pack installed that are not affected by this problem. I have copied the %systemroot%\%windir%\IME\IMJP\DICTS (all files were dated 4/10 or 11/2018) from one of those computers; overwriting the currently installed DICTS. Restart the computer and all worked for me. The non-1803(1709) version of the DICTS didn't worked at all.