C series dimmer function

I have an C127 MFD and the issue I am having is that the roto-knob adjusts the screen brightness in starts and fits going brighter when dimming and the reverse. It doesn't operate smoothly at all and at times it is just impossible to lower the brightness for night ops. Curious thing is that in other menu selections the knob seems to work just fine. I had updated the software recently to solve a depth sounder issue. The version I have is: v19.03-00036 Is it a hardware fault or software? Is it a field fix or do I need to send the unit in?

any insight would be helpful
many thanks in advance.
SQVFR