YIKES, DEFINITELY WOULDN'T WANT THAT TO HAPPEN WHEN YOU NEED IT MOST. I ALSO PERIODICALLY DUMP DEFENSIVE ROUNDS TO CONFIRM ZERO, REST/ROTATE MAGS, AND STAY FROSTY.
YOUR IDEA FOR THE MALFUNCTION SEEMS PLAUSIBLE, OR PERHAPS HUMIDITY COULD HAVE AFFECTED THE TOP FEW ROUNDS - DEPENDING ON YOUR CONDITIONS AND THE DURATION OF CARRYING SAID ROUNDS.