Please clarify some inconsistencies. Are you using 5.1 or master. You say 5.1 but the file you mention is master, although mentioning the red box around the date confirms master.
You say “date format is dd-mmm-yyyy (e.g. 29 sep 24)” but that example is “dd mmm yy” (not yyyy.) The red border around the date box is because either the required date is missing or something entered is not a valid date. In your last sentence you again repeat “date format of dd-mmm-yyyy (e.g. 29 sep 24)” but you again mix 2 and 4 digit years.
I would assume most if not all applications will use the system specified date format. That should be what KMyMoney is doing.
Finally, please state in more detail exactly what you are doing to get the red box around the date. I assume you are trying to enter a new transaction, but exact details will help us figure out exactly where the problem is.