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.
Windows – Fix Japanese IME after April 2018 update
imejapanesewindows 10windows-10-v1803
Best Answer
This seems to be related to the custom dictionary file (imjp15cu) as it gives an error whenever "edit" under custom dictionary is clicked in the advanced settings.
From a non-1803 system, copy the files in C:\Windows\IME\IMEJP\DICTS to the same location on the affect 1803 machine and reboot.