That's shown in the manual as "complement writing" mode. Maybe I misunderstood and thought that mem should be complemented? The logic unit is just a black box in the manual and it's behavior isn't well described.
Needs more testing -- I've just noticed that cursor disappears if this change is made, in vt240 as well.
There's also a video corruption issue involving scroll regions, easy to reproduce (output a screenful of text and then printf "\033[1;23r\033[23;1H\r\n") Worth a MT or github issue?