I recently observed the same thing on my AR2078.
I put a second grub screw in the breach behind the original, spaced so it's as far back as possible and misses the barrel O-ring. I also replace the original barrel grub screw with a 10-32 stainless one, and I snugged up the barrel band as much as possible. The barrel stays put now.
The original barrel grub screws are a soft steel, M5 thread. The 10-32 screw is almost identical, and being better steel with a well formed socket - you can get it nicely snugged up.