'Brownout detection' only works on DSMX RX's operating in DSM2 mode or the more recent versions of DSM2 RX's. If the OP is using a DSMX version of the DX5e (it has DSMX written on the faceplate), then brownout detection is not available.
The OP is using an AR6115e which only has failsafe on the throttle channel. The other channels will retain their last commanded positions. Your procedure for checking possible interference issues using failsafes will not work. Fortunately the AR6115e features a HOLD led that will tell you when the RX has lost communications with the TX. The OP could do the same range test and simply check for a flashing HOLD led. I routinely do this with my planes.