I am using ibus in openSUSE KDE 15.2. 
I can use ibus normally with an input like MN17 pre Latin but when I added
anthy to type in Japanese, it did not work, and neither could I switch to it
using the super + space key combination. 
Interestingly enough a Japanese input from mn17 trycode, worked, but that is
not used to type Japanese with a latin keyboard. 

I tested Chinese intelligent pinyin, it worked, I tested Korean hangul with a
latin keyboard, Russian translit phonetic, worked. 
Only Anthy does not work and the alternative kkc is not available for ibus to
install on openSUSE. 
I did not test it in mate or any other desktop, so maybe this a kde specific
bug, but it seems to affect anthy only.