Some changes in the firmware need to be flashed to both halves of a split keyboard, such as changes to the OLED display. Commonly, the slave half behaves unexpectedly when it doesn't have the most recent version of your firmware installed. To learn when you should flash your keyboard, read When do I need to flash my microcontroller?
Flash the firmware
Try flashing the firmware to the slave half first. Then unplug the keyboard, plug in the master side of the keyboard (often the left side), and see if the problem has been resolved.
Clean and rebuild
If you build your firmware from source and the keyboard then still behaves unexpectedly, try rebuilding your firmware:
Then flash both halves of the keyboard with the new firmware.