I don't know whether your plane is electric or internal combustion. It IS important to the question, you know. I come from the electric side of things. There brownouts are always not far from our thoughts, no matter what kind of radio we use. The battery elimination circuits in our electronic speed controls often are not robust enough to provide the amount of power our radios and servos will draw. If your radio and servos at peak draw, with four servos drawing power at the same time needs more amperage than your battery elimination circuit can provide, you get a voltage drop, no matter what condition your batteries are in. It also doesn't matter what kind of radio you have, you can brown them all out.

All you have to do is draw more amperage than your battery or battery elimination circuit can produce and your voltage will drop. When it does, there's a point where your radio quits working.

Have you considered the possibility that you didn't brown out at all, but were masked by some object opaque to radio waves, like a power line, possibly a fence, or even the earth itself? I've been shot down going behind a hill with 2.4 Ghz equipment. It is strictly line of sight and LOTS of things can break that line. Again, the brand of radio doesn't matter there.