There is a glitch in the UI for the Casement Window Settings. To get round the problem open up the Parameters tab and expand out the Sash Frame section and scroll down to the guilty setting as shown in the screenshot below.
You could also rummage around in the UI script of the object and find the offending call but I'm guessing you are new at this game and won't want to get into that
AC versions 3.41 to 25 (UKI Full 5005). Using AC25 5005 UKI FULL Mac OSX 10.15.7 (19G2021) Mac Pro-2013 32gbRam AMD FirePro D500 3072 MB graphics