Battery failsafe set up issue in MP

I was running through my failsafe set-ups on a couple of quads and have run into an odd problem.

First quad runs on 4S 5000mah battery and in MP the failsafe is set to 13.6v and 1000mah reserve (20%).

I then disconnected it from MP and connected my second, smaller quad which runs a 3s 4000mah battery.
I can set the failsafe voltage to 10.2v but MP will not let me set the reserve to 800mah. In fact it will not let me change from 1000, not even to 0 to disable the reserve mah.

Have I missed something? Or come across a problem?

Thanks.