Symbol that shouldnt be there!
 

I have had this problem several times when using katakana to write foreign words, here are 2 of them.

Morning = Mōningu = モーニング
when i write it, it comes out; モニング
the horizontal line symbol disappearsニ.

and cute = kyuto = キュート

ill take cute as an example: The dictionary i use says "キュート"
however when i write using the "katakana" feature on macOS it writes it
キュト where does the third letter go? that horizontal line always seems to be a problem..

what am i doing wrong?

slang
 

technically all words in katakana are slang, foreign, or just plain wierd. yor comp is like a virtual dictionary; sad to say, there is still no way of updating written slang for the japanese word document. it would be too much for one simple system like word. same when writing slang in english! u either get spell corrections, or if u spelled it right, u get a grammar correction telling u that u used it wrong in the sentence.
all i do is use the hyphen button when using katakana!:D
hope that helps!

Are you actually typing KYUTO to in order to get キュート？ If so, it'll never work. KYUTO is read キュト, so your PC is working. That's the limitation of romaji.

Try the 'minus' symbol (-) and change it to 'ー'.