And.........another thought after just reading your posts over again.
I'm wondering what the JRmatchboxes are doing to your voltage going to your reciever? What is the threshold that they deliver power. By that..I mean how much of a drain do the boxes actually allow ? I do know as servos are operated they are putting a load on a battery system, and servos will rob power from other sources that are not demanding it. I'm wondering if your matchboxes are allowing the power reaching your reciever to drop below the 3.8 volts.. I'm not saying it is a constant there, but if that voltage drops for a short time(milli-second?), could that be activating your battery-f/s mode.

What's JR's specs on these boxes? , and even more so, what's specs on JR's radios that might allow these to be more compatible there then with Futaba. I'm not saying that these are not compatible with Futaba, just areas to look at.

It might be interesting to see how it works without the matchboxes huh.

Have a look at all this, and get back to me in the morning.