I have just tested the MQTT publisher on the latest 7.10.02 firmware version and did not encounter any issues. If you are not using the latest firmware, please upgrade your device, as this should resolve the problem. You can download the firmware image here: RUT241 Firmware Downloads.
Regarding the password field, it is normal for it to appear blank after you press ‘Save & Apply.’ The password is saved and used, but it is not displayed for security reasons.