Are you using a single 24V supply, or +/-12V? If single supply, then you are probably running into an output swing issue. On page 4 of the data sheet, the typical output short circuit current is listed as 20mA on a 5V supply. The Output Saturation Voltage vs Load Current (Output High) graph on page 8 shows the output saturation voltage approaching 1V at 10mA output current so I would not be surprised to see it at 1.3V when the output is shorted and driving 20+mA with a 24V supply. Can you use a negative rail to allow more headroom and provide a load to limit the current?