I have been using Notes designer for years and have never come across this and it's driving me nuts.
I have a mail-in database where I use @Abstract on the body field to convert to to text and then extract values from this text into calculated text fields on the Memo form. These fields are set to type text and not Allow multiple values and use Native OS Style.
Rather than go into to much detail I attach a picture of how they display.
Why do they display like a Combo with arrows?