I learned that US Windows 10 Release 1803 has a problem with Japanese input. (to check your Windows 10 version
do this) See a better description of the problem
here.
It seems that the problem is with the Dictionary files. So you need to fix that by copying old dictionaries into your WINDOWS>IME>IMEJP>DICTS folder (or try something else.)
Many pages
like this one suggest:
"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."
Someone else suggested:
"I did in a different way. 'Open advanced settings', in 'Dictionary/Auto-tuning' tab, click Repair dictionaries button. I just worked afterwards. "
I got this fixed with a script provided by my company IT group but I can not share that here Having said that it seems that the script did what was suggested above, just added some older dictionary files to the IMEJP>DICTS folder as below:
The files highligted were added by the script and then I had to reboot my system.
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I wish you luck.