Apm mini 3.1 Can not set logging on

The quad flies pretty well, but I have problems with motor 4. It will not start like the other three even though I calibrated several times and even switched out with another esc and motor. I also had the motor get hot.

Anyhow, i tried to get a log to see what is happening, but I can not get the controller to log anything.
I set the bitmask parameter to log, it saves,but when I reconnect to mission planner I find the bitmask was reset to 0. The fc doesn’t seem to retain this parameter, but other parameters save ok.

Am I missing something or is this fc faulty?

Thanks,