I’m not aware of a flag on the device desc. that would turn that function on/off. Back in the days when the terminals were Coax cable connected, you had to put the terminal in setup mode for the display codes to be visible. And those codes you show are not familiar.
Screen Attributes for Color Displays
’21′ Green/Reverse Image
’23′ White/Reverse Image
’25′ Green/Underscore/Reverse Image
My guess would be a software problem. Are you using Client Access?